2D:4D digit ratio corresponds with fetal testosterone (and other hormones)

Correlation between 2D:4D digit ratio and testosterone

"Several studies now provide indirect support for the utility of 2D:4D ratio as an index of prenatal androgen concentrations. 

First, men and women with congenital adrenal hyperplasia (CAH), a condition leading to hyper-secretion of testosterone, have decreased 2D:4D ratios (Brown et al., 2002; Buck et al., 2003; Ökten et al., 2002; Rivas et al., 2014). Second, men with Klinefelter’s syndrome, a chromosomal abnormality associated with low androgen levels, have higher (i.e., feminized) 2D:4D ratio relative to healthy men (Manning et al., 2013). Third, there is some evidence indicating that 2D:4D ratio is masculinized in women with a male twin relative to women with a female twin, suggesting that hormone transfer from the male co-twin is masculinizing (Hiraishi et al., 2012; van Anders et al., 2006). Additionally, some studies indicate that testosterone concentrations from amniotic fluid is negatively correlated with 2D:4D ratio (Lutchmaya et al., 2004; Ventura et al., 2013). 

Perhaps the most compelling evidence for the validity of 2D:4D ratio as a marker of prenatal androgen exposure comes from work in rodents demonstrating that prenatal manipulations of testosterone as well as steroid hormone receptors modulate the development of 2D:4D ratio (Auger et al., 2013; Zheng and Cohn, 2011). Specifically, administration of anti-androgenic compounds results in feminized digit ratios (Auger et al., 2013). Additionally, inactivation of the androgen receptor during prenatal development decreases growth of the 4th digit, leading to a higher (i.e., feminized) 2D:4D ratio, whereas inactivation of the estrogen receptor α increases growth of the 4th digit, leading to a lower (i.e., masculinized) 2D:4D ratio. Similarly, testosterone administration during the prenatal period leads to a lower 2D:4D ratio, while estrogen administration leads to a higher 2D:4D ratio (Zheng and Cohn, 2011). For the purpose of the analyses conducted here we conceptualize 2D:4D ratio as a marker of prenatal androgen exposure. However, it should be noted that 2D:4D ratio can be influenced by multiple factors, including exposure to estradiol (Zheng and Cohn, 2011)."

(Gorka, Adam X., et al. "Anterior cingulate cortex gray matter volume mediates an association between 2D: 4D ratio and trait aggression in women but not men." Psychoneuroendocrinology 56 (2015): 148-156.)

"​Digit ratio—the relative length of index (2D) to ring (4D) finger—is a sexually dimorphic trait, with males typically having lower values of 2D:4D than females. The sex difference in 2D:4D is determined early in ontogeny, presumably by the end of the first trimester (...), and remains virtually unchanged through childhood and puberty (...). Although the sex difference is small, there is substantial variation in 2D:4D within each sex; this variation is associated with variations in fetal exposure to testosterone and estrogen, with testosterone increasing the relative size of the fourth digit and estrogen shortening it (...). Subsequent experimental studies of mice (...) and rats (...) have supported this model. Likewise, 2D:4D has been shown to deviate from human norms in the expected directions in such conditions as elevated testosterone (congenital adrenal hyperplasia [...]), androgen insensitivity (...), and low prenatal testosterone (...)."

(source: Digit Ratio (2D:4D) and Gender Inequalities Across Nations, Evolutionary Psychology, www.epjournal.net – 2014. 12(4): 757-768, John T. Manning, Department of Psychology, Northumbria University, Newcastle-upon-Tyne, UK, Bernhard Fink, Institute of Psychology and Courant Research Center Evolution of Social Behavior, University of Göttingen, Germany, Robert Trivers, Graduate Program in Ecology and Evolution, Rutgers University, New Brunswick, USA, retrieved 27/7/2016)

"A significant negative association between right 2D:4D ratio and FT (fetal testosterone)/FE (fetal estradiol) ratio [was found], which was independent of sex. These preliminary findings lend support to an association between low 2D:4D and high levels of FT relative to FE, and high 2D:4D with low FT relative to FE."

(source: 2nd To 4Th digit ratios, fetal testosterone and estradiol, in Early Human Development 77(1-2):23-8 · May 2004 , Svetlana Lutchmaya, Simon Baron-Cohen, Peter Raggatt, John T Manning)

"Studies on levels of testosterone (T) and oestrogen (O) in amniotic fluid following amniocentesis (...) have shown that, for the right hand only, there is a inverse relationship between the 2D:4D ratio and the T:O ratio which is independent of sex. Thus, individuals with higher 2D:4D ratios tended to show lower testosterone levels relative to oestrogen levels. No such relationship was observed for the left hand. Other investigations on females have noted positive correlations between 2D:4D ratios and levels of [luteinizing hormone] LH (right and left hands), oestrogen (right hand only) but not [follicle-stimulating hormone] FSH (...). These observations have led authors to suggest that the 2D:4D ratio in the right hand may be more sensitive to hormone ratios and this notion is supported by more recent studies showing that 2D:4D ratio is correlated with salivary oestradiol levels (...)."

(Mayhew, T. M., et al. "Human 2D (index) and 4D (ring) digit lengths: their variation and relationships during the menstrual cycle." Journal of anatomy 211.5 (2007): 630-638

"The ratio of the second digit (index finger) to the fourth digit (ring finger), herein referred to as 2D:4D, has been shown in infants to reflect their mothers’ amniotic ratios of testosterone to estradiol (Lutchmaya et al. 2004), which makes it a useful albeit noisy proxy for retrospectively measuring prenatal androgen exposure in humans (Cohen-Bendahan et al. 2005, Wong and Hines 2016). Though this biomarker cannot be ethically validated in humans experimentally, it has been shown that experimental manipulation of prenatal testosterone in rat fetuses results in a smaller 2D:4D claw-length ratio after the rats are born (Talarovičová et al. 2009). The effect is more pronounced on the right hand for both rodents (Talarovičová et al. 2009) and humans (Cohen-Bendahan et al. 2005), and right-hand 2D:4D has been shown to predict a level of conformity to aspects of stereotypical gender roles within a single-sex cohort (Atkinson et al. 2017, Brown et al. 2002)."

(Hejná, M., Ackerman, L., & Wallenberg, J. C. (2020). Attention to People Like You: a proposal regarding neuroendocrine effects on linguistic variation. Journal of Biolinguistics.)

Prenatal androgens may have greater effect on right hand digit ratio then on left hand digit ratio, postnatal androgens the opposite

"these data confirm the predominantly right-sided influence of androgens on digit length"

(Benderlioglu, Zeynep, and Randy J. Nelson. "Digit length ratios predict reactive aggression in women, but not in men." Hormones and behavior 46.5 (2004): 558-564.) 

Directional asymmetry of 2D:4D (left hand 2D:4D minus right hand 2D:4D) may be a better measure of prenatal testosterone than simply the right-hand 2D:4D.

"One possible explanation of this pattern, rather speculative indeed, could be that the right hand 2D:4D ratio reflects more the prenatal testosterone levels, while the left hand 2D:4D ratio is rather linked to postnatal testosterone levels (and thus is more prone to be influenced by environmental  factors (...))."

(Hodková, Hana, et al. "Higher perceived dominance in Toxoplasma infected men--a new evidence for role of increased level of testosterone in toxoplasmosis-associated changes in human behavior." Neuroendocrinology Letters 28.2 (2007): 110-114.)

"The sex difference in 2D:4D is larger in the right hand than in the left. (...) Right-hand 2D:4D might be a better indicator of prenatal androgenisation than left-hand 2D:4D."

(Hönekopp, Johannes, and Steven Watson. "Meta‐analysis of digit ratio 2D: 4D shows greater sex difference in the right hand." American Journal of Human Biology 22.5 (2010): 619-630.)

"These results suggest that left hand 2D:4D ratio is more susceptible to postnatal influences"

(Flegr, Jaroslav, Jitka Lindová, Věra Pivoñková, and Jan Havlíček. "Brief Communication: latent toxoplasmosis and salivary testosterone concentration—important confounding factors in second to fourth digit ratio studies." American Journal of Physical Anthropology: The Official Publication of the American Association of Physical Anthropologists 137, no. 4 (2008): 479-484.)

2D:4D ratio reflects prenatal balance of androgen (tesosterone) to estrogen to be precise, not exactly prenatal tesosterone as such

"Here we report that the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen signaling during a narrow window of digit development. Androgen receptor (AR) and estrogen receptor α (ER-α) activity is higher in digit 4 than in digit 2. Inactivation of AR decreases growth of digit 4, which causes a higher 2D:4D ratio, whereas inactivation of ER-α increases growth of digit 4, which leads to a lower 2D:4D ratio. We also show that addition of androgen has the same effect as inactivation of ER and that addition of estrogen mimics the reduction of AR."

(Zheng, Zhengui, and Martin J. Cohn. "Developmental basis of sexually dimorphic digit ratios." Proceedings of the National Academy of Sciences 108.39 (2011): 16289-16294.)

Correlation between 2D:4D ratio and other hormones

Right-hand (not left-hand) 2D:4D ratio positively associated with estradiol (but not progesterone) levels in women's saliva

"We tested the association between the index-to-ring finger length ratio (2D:4D) and ovarian steroid hormone concentrations measured over the course of a menstrual cycle in the saliva of 38 young women. Estradiol levels were positively associated with right-hand, but not left-hand, 2D:4D, and also with the difference between right- and left-hand 2D:4D. None of these measures predicted progesterone level."

(McIntyre, Matthew H., et al. "Index‐to‐ring finger length ratio (2D: 4D) predicts levels of salivary estradiol, but not progesterone, over the menstrual cycle." American Journal of Human Biology 19.3 (2007): 434-436.).

Ghrelin appears to be a further biochemical parameter in addition to the sex steroids which correlated with different digit-length ratios at least in boys

"The aim of this study was to investigate possible relationships between different right-hand finger-length ratios and different fasting hormone concentrations in young swimmers. Fifty-five young swimmers participated in this study (26 boys and 29 girls, aged 10–17 years). [...]
The following finger-length ratios were calculated: 1D:2D, 1D:3D, 1D:4D, 1D:5D, 2D:3D, 2D:4D, 2D:5D, 3D:4D, 3D:5D, and 4D:5D. All finger-length ratios were significantly higher in girls compared with boys. Ghrelin, leptin, testosterone in boys, estradiol in girls, insulin-like growth-factor I (IGF-I), IGFBP-3, and insulin were analyzed. Leptin and insulin concentrations were lower in boys compared with girls.[...]
In girls, estradiol correlated negatively with 2D:3D (r = −0.51) and 2D:4D (r = −0.49) finger ratios. In boys, ghrelin concentration correlated with most of the finger-length ratios (r = 0.37–0.40). In girls, the relationship of ghrelin with the 2D:3D (r = 0.45) and 2D:4D (r = 0.48) finger ratios was significant. In boys, but not in girls, IGF-I (r = 0.42) and IGFBP-3 (r = 0.44) correlated only with the 2D:4D finger ratio. Leptin and insulin did not correlate with the finger-length ratios. In boys, the most important hormone to characterize several finger-length ratios was ghrelin (13.7–15.6% variance accounted for). Ghrelin and testosterone together accounted for 20.3% (R 2 × 100) of the variance in the 2D:4D ratio. In girls, estradiol was correlated with the 2D:3D ratio (25.7%) and estradiol in combination with ghrelin with the 2D:4D ratio (30.0%). In conclusion, ghrelin appears to be a further biochemical parameter in addition to the sex steroids which correlated with different digit-length ratios at least in boys."

(Jürimäe, T., et al. "Relationships between finger-length ratios, ghrelin, leptin, IGF axis, and sex steroids in young male and female swimmers." European journal of applied physiology 104.3 (2008): 523-529.)

"Ghrelin [...] increase[s] body weight and fat mass [...].
[...]
In addition to its function in energy homeostasis, ghrelin also activates the cholinergic–dopaminergic reward link in inputs to the ventral tegmental area and in the mesolimbic pathway, a circuit that communicates the hedonic and reinforcing aspects of natural rewards, such as food and addictive drugs such as ethanol. [...] Ghrelin has been linked to inducing appetite and feeding behaviors. [...] [G]hrelin does not increase meal size, only meal number. Ghrelin injections also increase an animal's motivation to seek out food, behaviors including increased sniffing, foraging for food, and hoarding food."

(source: Wikipedia article on ghrelin, retrieved 5/8/2016)

One study calls for caution in relating prenatal testosterone to 2D:4D ratio

"Our results show the subtlety of androgen effects on digit ratio, and thus both confirm and refute claims about the value of 2D:4D. On the one hand, the higher mean ratios in individuals with CAIS (complete androgen insensitivity syndrome) than in typical men support the claim that digit ratio is influenced by androgen exposure. On the other hand, the modest size of the difference and the considerable within-group variability challenge the claim that digit ratio serves as a marker of individual differences in prenatal androgen exposure that can be related to other physical and psychological characteristics."

(Berenbaum, Sheri A., et al. "Fingers as a marker of prenatal androgen exposure." Endocrinology 150.11 (2009): 5119-5124.)

Assymetry in 2D:4D digit ratio correlates with circulating oestradiol in women

"Asymmetry in digit ratio [right 2D:4D ratio minus left 2D:4D ratio] correlated positively with oestradiol at each [menstrual] phase, as well as with average levels across the cycle. (...) No other significant correlations were observed between digit ratio variables [2D:4D and 2D:4Dr - 2D:4Dl] and circulating hormone levels [(progestrone and testosterone)]. Our results might suggest that low exposure to androgens and/or high exposure to oestrogens during gestation is a predictor of high oestradiol levels in naturally cycling females of reproductive age. However, considering that it was asymmetry in digit ratio, and not either right or left 2D:4D, that was a significant predictor, it is also possible that these effects reflect more general associations between bilateral asymmetry and circulating oestradiol levels."

(Richards, Gareth, et al. "Digit ratio (2D: 4D) and circulating testosterone, oestradiol, and progesterone levels across the menstrual cycle." Early Human Development 117 (2018): 68-73.)

2D:4D ratio may correspond to prenatal testosterone/estrogen ratio, but does not reflect current testosterone levels

"Consistent with meta-analytic conclusions (Honekopp et al., 2007), digit ratio and [baseline current] testosterone [measured from saliva sample] did not covary among males."

(Mead, Nicole L., et al. "Power Increases the Socially Toxic Component of Narcissism Among Individuals with High Baseline Testosterone.")

No correlation between testosterone in amniotic fluid or maternal plasma and mean 2D:4D (average of right-hand and left-hand 2D:4D) or D[R–L] (right-hand 2D:4D minus left-hand 2D:4D)

"Foetal sex hormones can have powerful and far-reaching effects on later phenotype. However, obtaining accurate measurements is difficult for ethical reasons, and researchers often employ proxy variables to examine their effects. The relative length of the second and fourth fingers (digit ratio or 2D:4D) is frequently used for this purpose, as it is hypothesized to index variance in prenatal androgen and oestrogen exposure. Most studies employing this method examine digit ratio for the right hand (R2D:4D) and/or left hand (L2D:4D), though the mean value (M2D:4D) (i.e., the average of R2D:4D and L2D:4D) and directional asymmetry (D[R–L]) (i.e., R2D:4D minus L2D:4D) are also commonly used. As no published studies have examined M2D:4D or D[R-L] in relation to testosterone measured from amniotic fluid, we conducted a secondary analysis of data published by Ventura et al. The sample comprises 106 mothers from Portugal who underwent amniocentesis during the second trimester and their neonates. Newborn M2D:4D was negatively correlated with amniotic testosterone in females (P<0.05) but not in males; no significant association was observed between amniotic testosterone and D[R–L] in either sex. In addition, we examined testosterone measured from maternal circulation during the second trimester, and found that it was not a significant predictor of M2D:4D or D[R–L] in male or female infants. Further research should aim to measure the ratio of testosterone to oestradiol present in amniotic fluid and maternal plasma, to examine whether either is a predictor of digit ratio variables at different stages of postnatal development."

(Richards, G., M. Gomes, and T. Ventura. "Testosterone measured from amniotic fluid and maternal plasma shows no significant association with directional asymmetry in newborn digit ratio (2D: 4D)." Journal of developmental origins of health and disease (2018): 1-6.)

Association between right-hand 2D:4D and estrogen recpetor polymporphism in school-aged boys

"The ratio of the lengths of the 2nd and 4th digits (2D:4D) is considered an index of prenatal exposure to androgen. Indeed, androgen receptors have been linked to digit length, but estrogen receptors are rarely investigated in this context. Thus, we investigated the association between estrogen receptor 1 (ESR1) genetic polymorphisms and 2D:4D in school-aged children. (...) Boys with the GG genotype at rs9340799 had a significantly lower 2D:4D in the right hand than boys with the AA/AG genotype (−0.96% lower, 95% confidence interval: −1.68 to −0.24). However, this association was detected only in boys born to non-smoking mothers. No significant differences were found between rs9340799 polymorphisms and 2D:4D among girls. There was also no link between 2D:4D and polymorphisms at rs2234693 and rs2077647. These data suggest that rs9340799 polymorphisms in ESR1 may contribute to digit length and 2D:4D."

(Nishimura, Yoko, Kimihiko Moriya, Sumitaka Kobayashi, Atsuko Araki, Fumihiro Sata, Takahiko Mitsui, Sachiko Itoh et al. "Association between ESR1 polymorphisms and second to fourth digit ratio in school-aged children in the Hokkaido Study." Steroids (2018).)

Digit ratios not only affected by prenatal testosterone (but probably by testosterone-to-estradiol ratios)

"The analysis of the variability in digit ratios provides additional information regarding the magnitude of the potential influence of androgens on digit ratios from both hands. Women with  CAIS  did  not  show  a  reduced  variability  in  digit  ratios,  which  would  be  expected  if androgens solely affect this measure. This is also a replication of the findings from the previ-ous digit ratio study in women with CAIS (Berenbaum et al. 2009) and again indicates that variations  in  digit  ratios  do  not  only  reflect  variations  in  prenatal  androgen  levels,  but  are also under the influence of other factors. In line with these findings, a meta-analysis of digit ratio studies has found that the magnitude of the digit ratio sex difference is smaller than the difference in amniotic testosterone levels (Hönekopp and Watson 2010), and classifica-tion of men and women according to their digit ratios was not very accurate (Berenbaum et al. 2009).Because estradiol receptors in women with CAIS are functional, estrogens might also play an  important  role.  It  has  been  suggested,  for  example,  that  sex  differences  in  digit  ratios result from differences in digit adiposity (Wallen 2009). Because both estradiol and andro-gen receptors have been found in fat tissue, these sex differences might be more strongly related to differences in testosterone-to-estradiol (T:E) ratios, instead of testosterone effects alone (Manning et al. 2014). This view is also in line with the negative association that was found between fetal T:E ratios and digit ratios at age two (Lutchmaya et al. 2004), although it is unknown whether this association is still found in adult digit ratios. (...)
Taken  together,  the  present  results,  along  with  results  from  previous  studies,  are  suggestive of an androgenic influence on both digit ratios and CEOAE amplitudes, while additional non-androgenic  factors  are  also  reflected  in  these  measures  obtained  in  adulthood.  Consequently,  both  measures  cannot  be  assumed  to  provide  a  reliable  indication  of  prenatal androgen  levels,  and  usage  of  these  measures  for  this  purpose  is  not  recommended,  as  it is likely to result in spurious associations. Further research is needed to clarify which other factors might be involved in the sexual differentiation of digit ratios and CEOAEs and what their exact timing and mechanisms of action would be."

(Van Hemmen, Judy, Peggy T. Cohen-Kettenis, Thomas D. Steensma, Dick J. Veltman, and Julie Bakker. "Do sex differences in CEOAEs and 2D: 4D ratios reflect androgen exposure? A study in women with complete androgen insensitivity syndrome." Biology of sex differences 8, no. 1 (2017): 11.)

Maternal urinary sexual hormone levels correlate with offspring 2D:4D in Titi monkeys

"The second‐to‐fourth digit (2D:4D) ratio is a sexually‐dimorphic biomarker for prenatal sex hormone exposure. We investigated whether titi monkeys (Plecturocebus cupreus) exhibit sexually‐dimorphic 2D:4D ratio, and whether variation in 2D:4D ratio correlates with maternal testosterone and estrogen levels during early pregnancy. Subjects were 61 adult titi monkeys (32 males, 29 females). For 26 subjects, maternal urine samples were collected approximately 15–20 weeks before birth and assayed for testosterone and estrone conjugate (E1C). Titi monkeys exhibited a human‐like pattern of sexual dimorphism in right‐hand 2D:4D ratio, with females exhibiting higher 2D:4D ratio than males (β = −0.29, p = 0.023). For left‐hand 2D:4D ratio, high levels of maternal E1C predicted low offspring 2D:4D ratio (β = −0.48, p = 0.009). For right‐hand 2D:4D ratio, high levels of testosterone (β = −0.53, p = 0.005) and testosterone‐to‐E1C ratio (β = −0.41, p = 0.028) predicted low offspring 2D:4D ratio. For 2D:4D ratio asymmetry (right‐hand – left‐hand), high levels of testosterone (β = −0.43, p = 0.03) and testosterone‐to‐E1C ratio (β = −0.53, p = 0.003) predicted low (right‐biased) asymmetry. This is the first report of sexually‐dimorphic 2D:4D ratio in New World monkeys, and the results support a growing literature suggesting prenatal sex hormones may modulate offspring 2D:4D ratio."

(Baxter, Alexander, et al. "Sexual Dimorphism in Titi Monkeys’ Digit (2D: 4D) Ratio is Associated with Maternal Urinary Sex Hormones During Pregnancy." Developmental Psychobiology.)

Study in rats shows no effect of prenatal testosterone treatment on 2D:4D in rats

"The effect of prenatal testosterone on the lengths of the dig-its  was  formerly  studied  in  both  mice  and  rats  (6,  8,  9).  These  studies have reported a positive correlation between testosterone concentration and the length of 4D (2, 6, 8, 9) resulting in a lower 2D:4D (9). In contrast, our results did not show any prolongation of 4D after prenatal testosterone treatment. There are only a few studies reporting no effect of prenatal T on the length of 2D (6, 8). In our study, both treatments resulted in the shortening of 2D, an effect also reported by Talarovičová et al (9). Huber et al (8) showed  no  effect  of  either  T  or  F  on  2D:4D.  Similarly,  in  our  study, there were no differences in the 2D:4D among treatments. These results are in contrast with previously published studies (6, 9) showing a relationship between the concentration of testoster-one and a lower 2D:4D.
(...)
The current study has limitations that should be pointed out. In the present study, the testosterone administration was restricted to the last week of gestation. The use of different doses of testoster-one during different periods of gestation may be more informative. Despite some limitations, to the best of our knowledge, this is the fi rst study analyzing the effect of prenatal hyperandrogenism on 2D:4D in relation to sex differences in laboratory rats using two different  methodologies.  The  results  of  the  anogenital  distance  measurements suggest that the dose of testosterone was adequate. Despite these fi ndings, in both sexes, prenatal testosterone, exog-enous or endogenous, had no effect on the 2D:4D. The results of our study can be explained by Manno et al (2008), who reported that the intrauterine formation of 2D and 4D is a complex process (31). This process can also be related to different testosterone me-tabolites  and  other  steroids  hormones,  not  only  to  testosterone.  In  conclusion,  the  results  of  our  study  demonstrate  that  the  2D:4D does not refl ect prenatal androgen exposure. The effect of prenatal testosterone was observed only in the decreased lengths of 2D and 4D compared to controls. Further studies focusing on the infl  uence of estrogen and other testosterone metabolites in dif-ferent periods of gestation on 2D:4D are necessary."

(Suchonova, M., V. Borbelyova, E. Renczes, B. Konecna, B. Vlkova, J. Hodosy, D. Ostatnikova, and P. Celec. "Does the 2nd and 4th digit ratio reflect prenatal androgen exposure?." Bratislavske lekarske listy 120, no. 9 (2019): 703-710.)

In water voles, 2D:4D digit ratio reliably associated with maternal androgen status

"The 2D : 4D digit ratio in males and females of a water vole (Arvicola amphibius L.) was measured. The analysis of the dependence of this index on gender and the morphophysiological characteristics of the mother (body weight after parturition, testosterone level during pregnancy) was carried out. A similarity was found between littermates in terms of finger index. It was found that the androgen status of the mother during pregnancy and the body weight of the mother after parturition reliably affect the 2D : 4D ratio. The conclusion about the possibility of using the 2D : 4D digit ratio in environmental population studies as a marker of prenatal hormone-related maternal effects was made."

(Nazarovaa, G. G., Proskurnyaka, L. P., & Yuzhika, E. I. (2020). Dependence of the Ratio of the Lengths for the Second and Fourth Fingers (2D: 4D) on the Prenatal Development Conditions in Water Vole (Arvicola amphibius L.). RUSSIAN JOURNAL OF DEVELOPMENTAL BIOLOGY, 51(1).)

Sex difference in 2D:4D not caused by differences in body size

"Introduction

Previous studies suggest that the second to fourth digit ratio (2D:4D) is higher in females than in males. Others claim that this is due to difference in overall body size. It is unclear which, if any, anthropometric measures may impact the ratio.

Aim

To investigate sexual dimorphism of the 2D:4D ratio in seven-year-old children, and associations between 2D:4D and other anthropometric measurements showing difference between girls and boys at this age.

Methods

A total of n=778 children were included in the present analyses. Measurements of finger lengths were performed by digital measurement in hand photocopies. The anthropometric measurements studied were birthweight, recumbent height at birth, present weight, height, BMI, body fat percentage and waist circumference. Sex differences in anthropometric measures (including 2D:4D) were analysed together with correlation between the studied measures.

Results

We found a small but significant difference in 2D:4D between the sexes: boys having a lower ratio than girls. There were no correlations between 2D:4D and any of the anthropometric measurements.ConclusionThere is a sexual dimorphism in 2D:4D that cannot be explained by boys being larger than girls. 2D:4D does not correlate to any of the anthropometric measurements assessed in the present study, and therefore they do not need to be considered confounders in furthers studies involving 2D:4D ratio."

(Julin, M. (2020). Second to fourth digit ratio-sexual dimorphism in seven-year-old children.)

 

2D:4D increases slightly with age in children, but rank order remains stable

"It has been hypothesised that the ratio between the length of the 2nd and 4th digits (2D:4D) is a correlate of prenatal sex steroids, and this relationship is strongest for the right hand. Furthermore, it has been suggested that 2D:4D is sexually dimorphic, the dimorphism is determined early, and 2D:4D among children is stable with growth. Here, we present the first longitudinal study of right and left hand 2D:4D. Our sample was 108 (54 males) Jamaican children. The first measurements were made in 1998 when mean age was 9.68 ± 1.39 years, and a second set of measurements were made in 2002. We found that: (i) there was a small increase in 2D:4D with age which was lowest in the right hand; (ii) 2D:4D was sexually dimorphic, the means for males and females differed in the same direction in the 1998 and 2002 samples, and the sex difference was significant in the 1998 but not in the 2002 sample; (iii) the correlation between the 1998 and 2002 measurements of 2D:4D was high, indicating that rank order of the ratio was stable across year groups; and (iv) the rate of change in 2D:4D did not differ significantly across year groups. We conclude that 2D:4D increases slightly with age in children with the effect less marked for the right hand (i.e. the hand which is likely to show the strongest association with prenatal steroids), 2D:4D is sexually dimorphic from an early age, and the rank order of 2D:4D is stable in children."

(Trivers, R., Manning, J., & Jacobson, A. (2006). A longitudinal study of digit ratio (2D: 4D) and other finger ratios in Jamaican children. Hormones and behavior, 49(2), 150-156.)

Study finds that digit ratio is not a good marker of individual differences in prenatal androgen exposure

"We report the strongest evaluation of the value of 2D:4D as a biomarker for early androgen exposure. Individuals with 46,XY karyotype but no effective prenatal androgen exposure due to complete androgen insensitivity syndrome had digit ratios that were feminized: they were higher than those of typical men and similar to those of typical women. Nevertheless, the effect was modest in size, and there was considerable within-group variability and between-group overlap, indicating that digit ratio is not a good marker of individual differences in prenatal androgen exposure."

(Berenbaum, S. A., Bryk, K. K., Nowak, N., Quigley, C. A., & Moffat, S. (2009). Fingers as a marker of prenatal androgen exposure. Endocrinology, 150(11), 5119-5124.)

Study findings casts doubt on the reliability of the 2D:4D measure as a biomarker for prenatal androgen exposure

• "We retrospectively study the second-to-fourth digit ratio (2D:4D) in youth with classical Congenital Adrenal Hyperplasia (CAH) and controls.
• We replicated previously reported associations of the 2D:4D with sex (lower 2D:4D in males) and age (increase in 2D:4D throughout development).
• The associations between 2D:4D and CAH diagnosis were small and statistically indistinguishable from zero, in the full sample and within sex.
• Our findings cast doubt on the reliability of the 2D:4D measure as a biomarker for prenatal androgen exposure in behavioral research."

(Nave, G., Koppin, C. M., Manfredi, D., Richards, G., Watson, S. J., Geffner, M. E., ... & Kim, M. S. (2020). No Difference in 2D: 4D Ratio between Youth with Elevated Prenatal Androgen Exposure due to Congenital Adrenal Hyperplasia and Controls. BioRxiv.)

 

Sex differences in the fingers of children are highly correlated with adult finger length ratios

"This paper reports the results of a study using serial radiographs to test for both sex differences in the fingers of infants and children and for a relationship between sex differences in the children and infant finger and adult finger length ratios. This is the first study using long-term serial data to evaluate the validity of finger length ratios as markers. We found not only that sex differences in finger length ratios arise prior to puberty, but that sex differences in the fingers of children are highly correlated with adult finger length ratios. Our results strongly encourage the further use of finger length ratios as markers of perinatal testosterone action."

(McIntyre, M. H., Ellison, P. T., Lieberman, D. E., Demerath, E., & Towne, B. (2005). The development of sex differences in digital formula from infancy in the Fels Longitudinal Study. Proceedings of the Royal Society B: Biological Sciences, 272(1571), 1473-1479.)

How does prenatal (fetal) testosterone alter the 2D:4D digit ratio?

Prenatal androgens increase 4D length (lowering 2D:4D ratio), prenatal estrogens decrease 4D length (increasing 2D:4D ratio) in mice

"Here we report that the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen signaling during a narrow window of digit development. Androgen receptor (AR) and estrogen receptor α (ER-α) activity is higher in digit 4 than in digit 2. Inactivation of AR decreases growth of digit 4, which causes a higher 2D:4D ratio, whereas inactivation of ER-α increases growth of digit 4, which leads to a lower 2D:4D ratio. We also show that addition of androgen has the same effect as inactivation of ER and that addition of estrogen mimics the reduction of AR. Androgen and estrogen differentially regulate the network of genes that controls chondrocyte proliferation, leading to differential growth of digit 4 in males and females. These studies identify previously undescribed molecular dimorphisms between male and female limb buds and provide experimental evidence that the digit ratio is a lifelong signature of prenatal hormonal exposure. Our results also suggest that the 2D:4D ratio can serve as an indicator of disrupted endocrine signaling during early development, which may aid in the identification of fetal origins of adult diseases."

(Zheng, Zhengui, and Martin J. Cohn. "Developmental basis of sexually dimorphic digit ratios." Proceedings of the National Academy of Sciences 108.39 (2011): 16289-16294.)

Figure above: Causal links between PT, PE, and 2D:4D. As described by Zheng and Cohn, receptors for androgen and estrogen are present on digits 2 and 4 of the mouse but are most plentiful on the fourth digit. Chondrocyte proliferation is stimulated by PT but arrested by PE. Compared with females, males have higher levels of PT relative to PE and longer fourth digits relative to second digits. Thus, the sex difference in 2D:4D is dependent on prenatal changes in the fourth digit caused by sex-dependent differences in the PT-to-PE ratio. Examples of male and female human hands are shown for comparison. 

(source: Manning, John T. "Resolving the role of prenatal sex steroids in the development of digit ratio." Proceedings of the National Academy of Sciences 108.39 (2011): 16143-16144.)

"For those who are curious about the causes of variations in 2D:4D, particularly as they relateto sex differences, testosterone has been shown to promote bone growth, including growth of the bones in the fingers (...). Once the gonads have differentiated into either testes (in males) or ovaries (in females) by mid-gestation, males are producing much higher levels of testosterone (...). As a result, their bodies grow more rapidly, although not all parts of the body, even specific fingers, grow at the same time (...). Ultimately, the length of the fourth finger is impacted more than the second finger by high prenatal testosterone regimens (...)."

(Ellis, Lee, Anthony W. Hoskin, and Malini Ratnasingam. "Testosterone, Risk Taking, and Religiosity: Evidence from Two Cultures." Journal for the Scientific Study of Religion 55.1 (2016): 153-173.)

Study on frogs concludes that in 'male-biased species' (where males have higher 2D:4D ratios than females) the dimorphic digit is digit II instead of IV

In female-biased species (like humans, mice and many other mammals) the 2D:4D digit ratio differs between individuals because Digit IV (4D) is more sensitive to sex-steroid hormones than Digit II (2D): prenatal testosterone makes 4D longer, prenatal estrogen makes 4D shorter.

In male-biased species (like frogs, toads, birds and also some primates, including the guinea baboons and the rhesus macaque), it's not that the direction of growth (elongation or shortening) is inversed, but it's Digit II (2D) which is more sensitive to sex-steroid hormones instead of Digit IV (4D).

(Lofeu, Leandro, Renata Brandt, and Tiana Kohlsdorf. "Phenotypic integration mediated by hormones: associations among digit ratios, body size and testosterone during tadpole development." BMC Evolutionary Biology 17.1 (2017): 175.)

Metacarpals and phalanges respond differently to prenatal androgens

"This study used metacarpals instead of phalanges to test the hypotheses that the metacarpal ratio (2Mc:4Mc) will be higher in females than males within a species and that the ratio would be more sexually dimorphic in species with high intermale competition compared to species with low intermale competition. (...) Second and fourth metacarpals were measured on skeletonized animals for six species with different mating strategies (M=monogamy; P=polygyny) and levels of intermale aggression (H=high; I=intermediate; L=low; F=female; M=male). (...) Sample sizes for each species including mating strategy and level of intermale aggression were as follows: Aotus azarae (M,L; 9F, 11M), Hylobates lar (M,L; 49F, 51M), Presbytis rubicunda (P,I; 18F, 19M), Trachypithecus cristatus (P,I; 31F, 18M), Alouatta seniculus (P,H; 17F, 19M), and Macaca fascicularis (P,H; 19F, 18M). Results comparing medians fail to reject the null hypotheses that the metacarpal ratio is the same between sex within the species and the ratios will be the same regardless of competition level. The sexes are non-significantly different in metacarpal ratio in A. seniculus, A. azarae, H. lar, and M. fascicularis; males have a significantly higher ratio than females in P. rubicunda, but females have a significantly higher ratio than males in T. cristatus. Results are not consistent with previous research on digit ratios indicating that metacarpals and phalanges may respond differently in their growth and PNAE [prenatal androgen exposure]."

(Hart, Emilee. "Metacarpal Ratio and Its Relation to Sexual Dimorphism in Primates with Different Mating Strategies." (2018).)

Left 2D:4D is more susceptible to postnatal influences than right 2D:4D

"These results suggest that left hand 2D:4D ratio is more susceptible to postnatal influences (...)."

(Flegr, Jaroslav, Jitka Lindová, Věra Pivoñková, and Jan Havlíček. "Brief Communication: latent toxoplasmosis and salivary testosterone concentration—important confounding factors in second to fourth digit ratio studies." American Journal of Physical Anthropology: The Official Publication of the American Association of Physical Anthropologists 137, no. 4 (2008): 479-484.)

Measuring the 2D:4D ratio

(Source: https://ibmathsresources.com/2013/05/05/finger-ratio-predicts-maths-ability/)

Measured mean values of 2D:4D ratios

Study of 464 university students (253 females) ages 18-22 in Duke University, North-Carolina, USA. 

Method: using NIH ImageJ software on digital scans.

• men
• right hand: 0.9594
• left hand: 0.9503
• women
• right hand: 0.9722
• left hand: 0.9650

(Gorka, Adam X., et al. "Anterior cingulate cortex gray matter volume mediates an association between 2D: 4D ratio and trait aggression in women but not men." Psychoneuroendocrinology 56 (2015): 148-156.)

Study on children of various ethnic backgrounds (798 children in the total sample: 90 Berbers, 438 Uygurs, 118 Han, and 152 Jamaicans). 

Method: photocopies.

• Han (China - oriental): right hand 0.954
• Berber (Morocco - caucasian): right hand 0.950
• Uygurs (Morocco - caucasian): right hand 0.946
• Jamaican (Afro-Caribbean): right hand 0.935

(source: "Sex and ethnic differences in 2 to 4 digit ratio of children", Manning, Stewart, Bundred, Trivers, in Early Human Development 80(2):161-8 · December 2004, retrieved July 1st 2016)

Study on 840 Nigerian Igbo and Yoruba adolescents (50% male). 

Method: caliper
Mean 2D:4D ratios:

• female:
• left: 0.95
• right: 0.97
• male:
• left: 0.94
• right: 0.96

(Gabriel, S. Oladipo, B. Fawehinmi Hakeem, I. Edibamode Ezon-Ebidor, A. Osunwoke Emeka, and S. Ordu Kenneth. "Second to fourth digit ratio in Nigerian Igbos and Yorubas." Scientific Research and Essays 4, no. 10 (2009): 1146-1148.)

Study on 83 adults 18-20 years old, 32 males and 51 females in Uttarakhand, India. 

Method: caliper. Mean 2D:4D ratios:

• female:
• left: 0.98
• right: 0.99
• male:
• left: 0.967
• right: 0.99

Broken down in left-handed and right handed participants:

We see above that

• females have higher 2D:4D ratios (left and right) than men
• the 2D:4D ratio is slightly lower on the dominant side (left-handed participants had a lower left 2D:4D than right 2D:4D, and right-handed participants a lower right 2D:4D than left 2D:4D)

("2D:4D Ratio in Adult Female and Their Comparison with Males: An Anthropometric Study from Northern India", Anamika Jaiswal, Ankit Kaushik, A K Singh, Government Medical College, Haldwani, Uttarakhand, India, Annals of Woman and Child Health, Vol. 3; Issue 4: 2017) 

Study on 419 Guatemalan women - very low 2D:4D for women

• female:
• left: 0.9337
• right: 0.9322

"The digit ratios for our sample are lower than those typically found in the literature. For the right hand, mean 2D:4D is 0.9322 (with a standard deviation of 0.0315); for the left hand the mean is 0.9337 (with a standard deviation of 0.0321)."

(Aycinena, Diego, and Lucas Rentschler. "Discounting and digit ratio: Low 2D: 4D predicts patience for a sample of females." Frontiers in behavioral neuroscience 11 (2018): 257.)

2D:4D ratios in Malay, Chinese and Indian males and females (150 participants)

(source: Nayak, Satheesha B., Dhiviah Nair, Vimal Ravi, and Ashwini P. Aithal. "A comparative study on digit ratio and hand patterns of three ethnic races of Malaysia." Egyptian Journal of Forensic Sciences 8, no. 1 (2018): 56.)

Measurements were made directly using a scale.

Concerning the right hand:

• males with a feminine (2D >= 4D) digit ratio:
• Malay: 40% (32 + 8)
• Chinese: 60% (16 + 44)
• Indian: 52% (20 + 32)
• females with a feminine digit ratio:
• Malay: 52% (40 + 12)
• Chinese: 36% (12 + 24)
• Indian: 76% (16 + 60)

Concerning the left hand:

• males with a feminine (2D >= 4D) digit ratio:
• Malay: 60% (32 + 28)
• Chinese: 44% (32 + 12)
• Indian: 48% (36 + 12)
• females with a feminine digit ratio:
• Malay: 24% (20 + 4)
• Chinese: 36% (24 + 12)
• Indian: 35% (11 + 24)

(Nayak, Satheesha B., Dhiviah Nair, Vimal Ravi, and Ashwini P. Aithal. "A comparative study on digit ratio and hand patterns of three ethnic races of Malaysia." Egyptian Journal of Forensic Sciences 8, no. 1 (2018): 56.)

Study on 86 students in North-East Pakistan 

• female:
• left: 0.979
• right: 0.977
• male:
• left: 0.958
• right: 0.958

"Of the 86 students, 42 (49%) were male and 44(51%) were female, with a mean age of 19.55±1.017 and19.86±0.905 years, respectively. The mean 2D:4D ratio for males on the left hand was 0.958±0.029 and for females it was0.979±0.363 (p=0.004). The ratio for the right hand for males  was  0.958±0.031  and  for  the  females  it  was0.977±0.035 (p=0.011)"

(Khan, Muhammad Zirik, Muhammad Hammad Malik, Roshaan Ahmad, Huma Saeed Khan, Shahid Hasan, and Zeeshan Ashraf Chaudhry. "2D: 4D digit ratios in males and females and its correlation with simple reaction time: A cross sectional Study at Combined Military Hospital (CMH), Lahore, Pakistan." JPMA (2019).)

Study on 100 adults (56 males) from the northwest of India

• female: 1.00
• male: 0.97

"The mean 2D:4D ratios in males were observed to be 0.97 by direct method and 0.96 by scan method. In females the ratio was 1.00 and 0.99 by direct and scan methods respectively"

(Jain, Monica, Usha Dhall, Suryamani Pandey, and Satyendra Jain. "Second to Fourth Digit Ratio (2D: 4D) in North-West Indians: Sexual Dimorphism." Journal of Anatomical Society of India 61, no. 2 (2012): 242-245.)

Study on 350 South Indian adolescents. 

Method: direct measurement.
Mean 2D:4D values:

• males: 0.95
• females: 0.99

(Kanchan, Tanuj, G. Pradeep Kumar, Ritesh G. Menezes, Prateek Rastogi, PP Jagadish Rao, Anand Menon, B. Suresh Kumar Shetty et al. "Sexual dimorphism of the index to ring finger ratio in South Indian adolescents." Journal of forensic and legal medicine 17, no. 5 (2010): 243-246.)

Study on 60 Greek men and 60 Greek women (randomly selected, age range 19-25 years). 

Method: calliper.

• female:
• left: 1.001
• right: 1.002
• male:
• left: 0.973
• right: 0.974

(Kyriakidis, Ioannis, and Paraskevi Papaioannidou. "Epidemiologic study of the sexually dimorphic second to fourth digit ratio (2D: 4D) and other finger ratios in Greek population." Collegium antropologicum 32, no. 4 (2008): 1093-1098.)

Comparative evaluation of 2D:4D ratios in males and in females in different studies

Author	Population	n	Gender	2D:4D
Manning et al (1998)[10]	English	400	M F	0.98 1.00
William et al (2000)[15]	American	108 146	M F	0.96 0.97
Swami et al (2013)[16]	Harayani Brahmins Kashmiris pandits	150 150	M F M F	0.95 0.98 0.99 1.001
Present study	Tibeto-Nepalese Indo-Nepalese	200 200	M F M F	1.004 0.99 1.003 1.004

Abbreviations: 2D:4D, second to fourth digit ratio; F, female; M, male.

(Shah, Sandip, Sarun Koirala, and Laxman Khanal. "Relationship of Handedness with Second to Fourth Digit (2D: 4D) Ratio and its Role in Sexual Dimorphism in Tibeto-Nepalese and Indo-Nepalese Adult Population of the Dharan Municipality, Sunsari District of Eastern Nepal: An Anthropometric Study." Journal of Morphological Sciences (2019).)

Study on 269 adolescents of 18–19 in South-East India. 

• males and females: 0.97

Method: calliper.

"Mean 2D: 4D ratio of the study population was 0.97±0.082. There was no significant difference in mean 2D: 4D ratios (Males=Females) (...). "

(Jeevanandam, Saravanakumar, K. Muthu Prathibha, and K. Raman. "Relevance of 2D: 4D Ratio as a Marker of Depression in Adolescents of a South Indian Medical College-A Cross Sectional Study." International Journal of Physiology 7.2 (2019): 80-85.)

Study on 100 medical students in South-west-India

• female:
• left: 0.98
• right: 0.97
• male:
• left: 0.98
• right: 0.97

Hundred first year medical students (n=100) (males= 50 and females =50) of S. N. Medical college, Bagalkot.

Measurement method: digital calliper

"In Males, the ratio varied from 0.89 to 1.04 with mean and standard deviation of 0.97 ± 0.03 on right side and on left side it varied from 0.85 to 1.03 with mean ± SD of 0.98 ±0.04. While in females ratio varied from 0.91 to 1.11 with Mean ± SD of 0.97±0.04 on right side and on left side it varied from 0.83 to 1.12 and Mean ± SD of 0.98 ±0.04 on left side. (...) When all the cases below and equal to 0.97 were considered as males and those above 0.97 were considered as females, one could accurately determine the sex up to 90% accuracy."

(Sheelavant, Santosh, and Manjula Patil. "Role of 2nd Digit: 4th Digit Ratio (2D: 4D Ratio) in Establishing the Biological Profile of an Individual." Medico-Legal Update 19.2 (2019): 175-179.)

Study on 247 students (180 female, 67 male) of College of Health Technology, Ilese Ijebu, Ogun State, Nigeria

Method: calliper and hand scan
Results (mean 2D:4D):

• males:
• right hand: 0.95 (calliper), 0.94 (hand scan)
• left hand: 0.96 (calliper), 0.94 (hand scan)
• females:
• right hand: 0.95 (calliper), 0.95 (hand scan)
• left hand: 0.96 (calliper), 0.95 (hand scan)

 ("Digital Ratio and Academic Performances of College Students", T. Adenowo, and O. Oladokun.
International Journal of Recent Innovations in Medicine and Clinical Research 1 (1): 29-36 (August 2019))

Study on 100 subjects (50 male, 50 female) in North-Pakistan

Method: "compass" and ruler
Results (mean R2D:4D):

• males: 0,991
• females: 0,995

 ("THE GENDER DIFFERENCE OF 2nd TO 4th DIGIT LENGTH AND LENGTH RATIO (2D:4D) IN LOCAL POPULATION", Foundation University Med J 2014; 1(1):11-14, Syed Muarraf Hussain, Saadat Ali Khan, Syeda Maria Ali, SyedAbdul Mughees Hussain)

Study on the Bini people in South Nigeria

400  subjects  (225  males,  175  females), ages 18-60

"The  males  had  the  Left  Digit  Ratio  0.98±0.03,  Right  Digit Ratio 0.97±0.02. While in the females the Left Digit Ratio was 0.98±0.05, Right Digit Ratio 0.99±0.04."

(Paul, John N., Deborah A. Akinola, and Chibuike Obiandu. "A Descriptive Study of Gender Difference in Digit Ratio among Bini Ethnic Group of southern Nigeria.")

Germans have significantly higher mean 2D:4D than Chinese

"The German sample had significantly higher 2D:4D ratios than the Chinese sample but in both samples males had significantly lower 2D:4D ratios than females."

(Wernicke, J., Zabel, J. T., Zhang, Y., Becker, B., & Montag, C. (2020). Association between tendencies for attention-deficit/hyperactivity disorder (ADHD) and the 2D: 4D digit ratio: a cross-cultural replication in Germany and China. Early Human Development, 143, 104943.)

1,5 year old children in Japan

• Males:
• left: 0.909 ± 0.048
• right: 0.938 ± 0.055
• Females
• left: 0.913 ± 0.049
• right: 0.937 ± 0.049

(Yamada, Y., Ebara, T., Yamaguchi, C., Miyachi, T., Shoji, N., Matsuki, T., ... & Kamijima, M. (2020). Sexual difference in 2nd-to-4th digit ratio among 1.5-year-old Japanese children: A cross-sectional study of Aichi regional adjunct cohort of the Japan Environment and Children's Study (JECS-A). Early Human Development, 105050.)

 

 

Mothers, daughters and sons in Ghana

"This was a cross-sectional study from December 2020 to April 2021 involving 272 mothers, their daughters (n = 132) and their sons (n = 140). The right (2D:4DR) and the left (2D:4DL) digit ratios were measured using computer-assisted analysis. (...)

The mean ± SD 2D:4DR were 0.94 ± 0.04, 0.91 ± 0.04 and 0.90 ± 0.04 respectively for mothers, daughters and sons. The mean ± SD 2D:4DL was 0.93 ± 0.04, for mothers, 0.92 ± 0.05 for daughters and 0.92 ± 0.05 for sons. (...) The 2D:4DR of mothers was positively correlated with that of daughters (r = 0.332, P = 0.000) and that of sons (r = 0.233, P = 0.008). There are significant relationships between digit ratios in a mother–child population."

(Banyeh, M., Amidu, N., & Quaye, L. (2021). Second to fourth (2D: 4D) digit ratio and their relationships among a mother and child population in Ghana. Scientific Reports, 11(1), 1-11.)

Sexual dimorphism in the second-to-fourth digit ratio is not an artifact of differences in height

"Recently, a number of authors have claimed that sexual dimorphism in the second-to-fourth digit ratio (2D:4D) is simply dependent on digit length and is an artifact of allometry. The goal of our study is to verify the validity of these assumptions. The study sample comprised 7,582 individuals (3,802 men and 3,780 women) from three large world populations: Europeans (n = 3043), East Africans (n = 2844), and Central Asians (n = 1695).

(...)

[T]he 2D:4D magnitude and its sexual dimorphism remained stable throughout the ontogeny. To test for an allometric effect on 2D:4D, the average digit lengths were calculated. Both sex and population origin were permanent reliable predictors of 2D:4D, whereas average digit length was not. Height was applied as another measure of allometric effect on the limited sample (≤ 30 years) from the European population, along with sex and age. No allometric effect was observed in this case. We conclude that sex differences in 2D:4D are not an artifact of allometry."

(Butovskaya, M., Burkova, V., Apalkova, Y., Dronova, D., Rostovtseva, V., Karelin, D., ... & Batsevich, V. (2021). Sex, population origin, age and average digit length as predictors of digit ratio in three large world populations. Scientific reports, 11(1), 1-17.)

Self-reported 2D:4D ratios higher (and sometimes very incorrect) compared to measurements from photocopies

"In the present study, we compared 2D:4D from self-reported finger lengths measured directly from the fingers (S-R 2D:4D) and experimenter-measured finger lengths from photocopies of the fingers (photo 2D:4D). There were 329 participants (77 men, 252 women) recruited from a first-year undergraduate psychology pool. Compared to photo 2D:4D, (1) S-R 2D:4D tended to include some extreme values; (2) S-R 2D:4D was higher; (3) S-R 2D:4D showed weak similarities [with photo 2D:4D] which increased when extreme values of S-R 2D:4D were removed; (4) photo 2D:4D and S-R 2D:4D showed lower values for males compared to females but the dimorphism was significant for the former but not for the latter."

(source: A Comparison of Finger 2D:4D by Self-Report Direct Measurement and Experimenter Measurement from Photocopy: Methodological Issues, Noreen Caswell, John T. Manning; 4 February 2007, Arch Sex Behav, DOI 10.1007/s10508-007-9202-3, retrieved 27-7-2016)

Smartphone acceptable as instrument for measuring 2D:4D ratio, if used correctly

"This study explores the feasibility of using a Smartphone as an instrument for measuring finger length ratios. The approach taken in this study is to use the Smartphone camera to take freehand photos of the hand which is subsequently subjected to image analysis. [...] Experimental evaluations show that the most accurate measurements are achieved by resting the hand on a surface with the palm facing up. These results are comparable to those achieved with conventional procedures with an error of 1%."

(source: Measuring 2D:4D finger length ratios with Smartphone cameras, Frode Eika Sandnes ; Institute of information Technology, Faculty of Technology, Art and Design, Oslo and Akershus University College of Applied Sciences, Norway, 2014, DOI 10.1109/SMC.2014.6974161)

Computer software better than caliper better than ruler for measuring 2D:4D ratio (but all acceptable)

"Using different indirect methods (plastic ruler, caliper, computer software), three independent raters measured finger lengths of 60 participants. Generally, measurement precision [...] was acceptable for each method. However, precision estimates were highest for the computer software, indicating excellent measurement precision. Estimates for the caliper method were somewhat lower followed by ruler which had the lowest precision."

(source: Am J Hum Biol. 2009 Mar-Apr;21(2):188-91. doi: 10.1002/ajhb.20843., Comparing indirect methods of digit ratio (2D:4D) measurement, Kemper CJ1, Schwerdtfeger A.)

2D:4D increases slightly with age in children, rank order is stable

"We conclude that 2D:4D increases slightly with age in children with the effect less marked for the right hand (i.e. the hand which is likely to show the strongest association with prenatal steroids), 2D:4D is sexually dimorphic from an early age, and the rank order of 2D:4D is stable in children."

(Trivers, Robert, John Manning, and Amy Jacobson. "A longitudinal study of digit ratio (2D: 4D) and other finger ratios in Jamaican children." Hormones and Behavior 49.2 (2006): 150-156.)

Inter-observer reliability in 2D:4D measures higher using photocopies than caliper

"Photocopying has been shown to be effective in previous human 2D:4D studies (Manning et al. 2001; Peters et al. 2002; Rahman & Wilson, 2003). The method produces measurements which correlate well with those obtained directly from the hands with callipers (Robinson & Manning, 2000) and tends to give lower 2D:4D ratios but with enhanced inter-observer reliability (Peters et al. 2002; Manning et al. 2005)."

(Firman, Renée C., et al. "Are body fluctuating asymmetry and the ratio of 2nd to 4th digit length reliable predictors of semen quality?." Human Reproduction 18.4 (2003): 808-812.)

Wrist position alters 2D:4D measures using photocopies

"The pilot study was designed to test the null hypothesis that wrist position (abducted, adducted or straight) does not alter 2D or 4D length measurements for an individual. Twelve independent participants (n = 6 males and n = 6 females) were selected randomly from a set of medical undergraduates in the library of the University of Nottingham Medical School. The purpose of the study was explained and each student consented to partake. Any student who had fractured, suffered trauma to, or had arthritis in, their 2D or 4D was excluded and students were asked to remove any rings from these digits.

The photocopier contrast was altered initially so that the proximal crease and distal finger tip were clearly visible. Three photocopies were taken from each student, the first with both hands in full abduction (radial deviation) at the wrist, the second with both hands in full adduction (ulnar deviation) and the third with a straight wrist, such that a line drawn along the length of the third digit extended between the radius and the ulna [the two long bones in the forearm]. In each position, the lengths of 2D and 4D were measured from the midpoint of the proximal crease, to the distal finger tip point, using digital callipers accurate to 0.01 mm. Differences between the 2D and 4D measurements were calculated for each of the three photocopies. For any individual, the minimum difference between photocopies was 0.05 mm and the maximum difference was 3.00 mm.

Despite the small sample sizes, significant differences between length measurements of the same digit were found for the three different hand positions. These indicated that abduction at the wrist favoured the 2D and adduction favoured the 4D. In a larger study (see Peters et al. 2002), similar significant differences were established."

(Firman, Renée C., et al. "Are body fluctuating asymmetry and the ratio of 2nd to 4th digit length reliable predictors of semen quality?." Human Reproduction 18.4 (2003): 808-812.)

2D:4D ratio in women not affected by menstrual cycle 

This study:

"In [righthanded women age 19-21] not taking oral contraceptives, there were significant differences over the menstrual cycle in both hands."

(Mayhew, T. M., et al. "Human 2D (index) and 4D (ring) digit lengths: their variation and relationships during the menstrual cycle." Journal of anatomy 211.5 (2007): 630-638.

was contradicted by this larger study:

"No differences in 2D:4D value across the menstrual cycle were detected either when right-hand, left-hand, and mean 2D:4D for both hands were analysed, nor when difference in the 2D:4D value between hands (Dleft-right) was evaluated. We documented that 2D:4D is independent of the phase of the menstrual cycle and these findings suggest that among naturally cycling women, a value of 2D:4D can be reliably obtained from measurements taken during any day of the menstrual cycle."

(Klimek, Magdalena, Urszula M. Marcinkowska, and Grazyna Jasienska. "Value of digit ratio 2D: 4D, a biomarker of prenatal hormone exposure, is stable across the menstrual cycle." Early Human Development 110 (2017): 21-24.)

Gender differences in 2D:4D ratios focussed on dominant hand

"[G]ender differences in [2D:4D] ratios appear to be focussed on the dominant hand."

(Mayhew, T. M., et al. "Human 2D (index) and 4D (ring) digit lengths: their variation and relationships during the menstrual cycle." Journal of anatomy 211.5 (2007): 630-638.

Right-hand 2D:4D more reliable indicator of prenatal androgenisation than left-hand 2D:4D

"Right-hand 2D:4D might be a better indicator of prenatal androgenisation than left-hand 2D:4D. The view that 2D:4D has allometric properties (Kratochvil L, Flegr J. 2009. Differences in 2nd to 4th digit length ratio in humans reflect shifts along the common allometric line. Biol Lett 5:643-646.) is not supported. Bone lengths contribute to the sex difference in 2D:4D. In addition, there might be a sex difference in fingers' soft tissue, which should be investigated."

(Hönekopp, Johannes, and Steven Watson. "Meta‐analysis of digit ratio 2D: 4D shows greater sex difference in the right hand." American Journal of Human Biology 22, no. 5 (2010): 619-630.)

2D:4D is not age-dependent

"2D:4D is not age‐dependent"

(Manning, John T., and Bernhard Fink. "Sexual dimorphism in the ontogeny of second (2D) and fourth (4D) digit lengths, and digit ratio (2D: 4D)." American Journal of Human Biology 30, no. 4 (2018): e23138.)

Palmar crease in men is differently placed than in women

"The results show that men have lower palmar-to-dorsal digit ratios than women, and this effect is significantly stronger for digit 2 than for digit 4. Thus, the present study supports the likelihood that differential placing of flexion creases in the digits contributes to the occurrence of sex differences in palmar 2D:4D."

(Kumar, Sanjay, Martin Voracek, and Maharaj Singh. "Differential placing of flexion creases contributes to sex differences in the second-to-fourth digit ratio (2D: 4D)." Frontiers in Endocrinology 10 (2019): 537.)

In Russia, the 2D:4D ratio of men increases from East to West

"The study was conducted on 263 young men (age range 17-30 years), including Russians, Armenians, Ob-Ugric and Buryats. The results revealed significant population differences in 2D:4D ratios on both hands, with lowest ratios for Asian sample, increasing in populations to the West. (...) Positive association between 2D:4D ratios on the right hand and Agreeableness in men with no respect to population origin was detected. This relationship becomes stronger, when controlling for aggressiveness."

(Victoria, Rostovtseva, Butovskaya Marina, and Mkrtchjan Ruzan. "2d: 4d, Big Fives and Aggression in Young Men of Caucasian, Ural and Asian Origin." Social Evolution & History 18, no. 1 (2019).)

Elbow carrying angle correlates with 2D:4D ratio in men and women

"Abstract: 

Background: The acute angle produced medially when the axis of the forearm deviates laterally from the long axis of the humerus, with the arm extended and the palm facing forward, is referred to as the carrying angle. Variations in the angle have clinical as well as pathological significance. 

(...)

Methods: 195 subjects with age varying from 12 to 32 years were studied. Carrying angle was measured using goniometer. Length of second and fourth digit was measured from distal palmar crease to tip of finger. 

Results: The result of the study showed that carrying angle is more in females than males on both sides. Lower values of carrying angle seen in males is strongly associated with longer ring finger and higher values of carrying angle seen in females associated with a longer index finger. There was only a moderately positive correlation between the carrying angle and fingertip lengths." 

(Biradar, Ravi Kumar, and H. M. Adarsha. "Study of elbow carrying angle and its relationship with the length of second and fourth digit." (2019).)

Factors influencing (prenatal) testosterone and 2D:4D ratio

Environmental pollution can disturb steroidgenesis

"During the past 70 years, there was an average production of 1000–2000 new chemical compounds per year, most of them being harmful or environmentally toxic. After disasters such as those in Seveso, Italy, in 1976 or Bhopal, India, in 1984, where thousands of people were harmed and 200,000 had to be epidemiologically monitored and after estimations that about 40% of annual deaths are related to chemical pollution, the health risk of these chemicals became evident and stricter laws came into effect. These different environmental pollutants such as plasticizers, pesticides, or heavy metals can cause direct damage to cells in the germinal epithelium such as the Sertoli cells leading to low sperm counts. The effect can also be on the Leydig cells where steroidogenesis is disturbed. Similar effects will take place if the hypothalamus–pituitary–gonadal axis is affected by endocrine disruption. As a result, changes in the sex ratio, congenital malformation such as hypospadias, or cancers can develop. Therefore, there is an urgent need to reduce environmental pollution to ensure future generation's well-being."

(Giwercman, Aleksander, and Yvonne Lundberg Giwercman. "Environmental factors and testicular function." Best practice & research Clinical endocrinology & metabolism 25.2 (2011): 391-402.)

See also the Wikipedia article on Xenoestrogens.

"Nonylphenol (NP) and di-n-butyl phthalate (DBP), two well-known EDCs, are high-production volume chemicals widely distributed in the environment. NP and phthalates can leach from products that contain them and accumulate in the environment [2]. Such environmental toxicants can be taken up by humans through ingesting contaminated food or water. Recent studies suggest that environmental levels of NP may exert estrogenic effects in humans and wild animals, disturbing the balance of hormone secretion and cytokine network at the maternal-fetal interface [3], [4]. Exposure to DBP, also referred to as antiandrogen, between gestation days (GD) 12 and 21 disrupts sexual development in rats, leading to decreased anogenital distance, reproductive tract malformations, seminiferous tubule degeneration, interstitial cell hyperplasia and adenoma in the testis, as well as small reproductive organs in male F1 rats [5]. Monobutyl phthalate (MBP) [5], [6], the active monoester metabolite of DBP, can inhibit the fetal testosterone synthesis resulting in anti-androgenic effects [7]–[9]."

(Hu, Yang, et al. "Antagonistic effects of a mixture of low-dose nonylphenol and di-n-butyl phthalate (monobutyl phthalate) on the Sertoli cells and serum reproductive hormones in prepubertal male rats in vitro and in vivo." PloS one 9.3 (2014): e93425.)

Glyphosate at "safe" dosage linked to masculinization of both male and female rats

"The present pilot study examine whether exposure to GBHs at the dose of glyphosate considered to be “safe” (the US Acceptable Daily Intake - ADI - of 1.75 mg/kg bw/day), starting from in utero life, affect the development and endocrine system across different life stages in Sprague Dawley (SD) rats. (...)
The present pilot study demonstrate that GBHs (Glyphosate-based herbicides) exposure, from prenatal period to adulthood, induced endocrine effects and altered reproductive developmental parameters in male and female SD (Sprague Dawley ) rats. In particular, it was associated with androgen-like effects, including a statistically significant increase of AGDs (anogenital distance) in both males and females, delay of FE (Age at first estrous) and increased testosterone in female."

(Manservisi, Fabiana, Corina Lesseur, Simona Panzacchi, Daniele Mandrioli, Laura Falcioni, Luciano Bua, Marco Manservigi et al. "The Ramazzini Institute 13-week pilot study glyphosate-based herbicides administered at human-equivalent dose to Sprague Dawley rats: effects on development and endocrine system." Environmental Health 18, no. 1 (2019): 15.)

Pregnant women's exposure to Triclosan increases offspring's testosterone levels (especially in males)

"Triclosan (TCS) has been widely detected in pregnant women. [...] Prenatal TCS exposure was associated with increased testosterone concentrations in cord blood in a dose-dependent manner. [...] Of note, prenatal TCS exposure was associated with increased testosterone and decreased E2 (estradiol) concentrations in cord blood among male infants."

(Wang, Caifeng, et al. "Impacts of prenatal triclosan exposure on fetal reproductive hormones and its potential mechanism." Environment International (2017).)

"Triclosan [...] is an antibacterial and antifungal agent found in some consumer products, including toothpaste, soaps, detergents, toys, and surgical cleaning treatments. [...] Humans are exposed to triclosan through skin absorption when washing hands or in the shower, brushing teeth, using mouthwash or doing dishes, and through ingestion when swallowed. When triclosan is released into the environment, additional exposure to the chemical is possible through ingesting plants grown in soil treated with sewage sludge, or eating fish exposed to it."

(Wikipedia, Triclosan)

Prenatal alcohol reduces testosterone and increases left-hand digit ratios in rats

"Since prenatal alcohol can reduce testosterone in rats, the effects of prenatal alcohol were measured on rat digit length. [...] Rats exposed to [a high dose] 6 g/kg of ethanol had smaller digits than controls on both forepaws and higher digit ratios than controls on the left forepaw. Rat digit ratios differ between the sexes, and prenatal alcohol exposure affects digit ratios. The results are consistent with a perinatal disruption of testosterone levels by alcohol and/or of testosterone's effects on digit length and ratios."

(McMechan, Andrew P., et al. "Effects of prenatal alcohol exposure on forepaw digit length and digit ratios in rats." Developmental psychobiology 45.4 (2004): 251-258.)

Gene pools affect 2D:4D ratio, geopgraphical latitude does not

"Studies have reported a difference in this ratio between Caucasian males in Britain and in the U.S.: higher average 2D:4D ratios were obtained in Britain. This raises the question of whether differences among different Caucasian gene pools were responsible or whether some environmental variable associated with latitude might be involved (e.g., exposure to sunlight or different day-length patterns). This question was explored by examining 2D:4D ratios for an Australian adolescent sample. The Australians were predominantly of British ancestry, but lived at distances from the equator more like those of the U.S. studies. The Australian 2D:4D ratios resembled those in Britain rather than those in the U.S., tending to exclude hypotheses related to latitude and making differences in gene pools a plausible explanation."

(source: Population Differences in Finger-Length Ratios: Ethnicity or Latitude?, John C. Loehlin, Ph.D., Dennis McFadden, Ph.D., Sarah E. Medland, Ph.D., and Nicholas G. Martin, Ph.D., Archives of Sexual Behavior, Vol. 35, No. 6, December 2006, pp. 739–742, retrieved July 1st 2016)

"We concluded that the sex differences in Chinese 2D:4D are consistent with the results from western studies, and that genetic pool differences rather than latitude-related environmental variables affect the 2D:4D in China. In addition, 2D:4D measurements obtained by different digit measurement techniques should not be combined within one study, or used together in comparative studies."

(source: The digit ratio (2D:4D) in China: A meta-analysis, Yin Xu, Yong Zheng, American Journal of Human Biology, 2014, DOI: 10.1002/ajhb.22639)

Prenatal alcohol exposure related to elevated testosterone in adolescents

"Prenatal alcohol exposure was related to elevated testosterone concentrations for males and females but not to changes in Tanner stages or age at menarche ( = first menstrual cycle). [...] This study was the first to show a relation between prenatal alcohol exposure and increased testosterone during adolescence and evidence of decreased testosterone responsiveness in tissues related to pubertal development."

(Carter, R. Colin, et al. "Effects of prenatal alcohol exposure on testosterone and pubertal development." Alcoholism: Clinical and Experimental Research 38.6 (2014): 1671-1679.)

(The Tanner scale (also known as the Tanner stages) is a scale of physical development in children, adolescents and adults. The scale defines physical measurements of development based on external primary and secondary sex characteristics, such as the size of the breasts, genitals, testicular volume and development of pubic hair.)

(Wikipedia, Tanner scale)

Inbreeding associated with lower, more masculine-type 2D:4D ratios in offspring

"Across hands and sex, consanguineous parentage was consistently associated with lower, more masculine-typical, digit ratios."

(Özener, Baris, Peter L. Hurd, and İzzet Duyar. "Inbreeding is associated with lower 2D: 4D digit ratio." American Journal of Human Biology 26.2 (2014): 183-188.)

"Our study from large population in India concludes that consanguinity increases the risk for ASD with an odds ratio of 3.22." 

(Mamidala, Madhu P., et al. "Consanguinity in India and Its Association With Autism Spectrum Disorder." Autism Research 8.2 (2015): 224-228.)

(And ASD has been linked with lower 2D:4D ratios and high prenatal testosterone, see the items on autism, ASD and Asperger's in Effects of (fetal) testosterone on brain and behavior).

In sheep, mother's nicotine exposure increases prenatal testosterone of fetuses

"We conclude that prenatal nicotine exposure increases plasma testosterone levels chronically in adolescent female rat offspring and acutely in both male and female ovine fetuses."

(Smith, Lynne M., et al. "Prenatal nicotine increases testosterone levels in the fetus and female offspring." Nicotine & tobacco research 5.3 (2003): 369-374.)

Correlation between physical and sexual maternal activity during pregnancy, and masculine traits in female offspring

"By and large, as maternal physical activity increased, feminine mannerisms decreased and masculine mannerisms increased in the offspring, particularly for females. Maternal physical activity was also associated with increased upper- and lower-body strength and especially with adult height among offspring. Sexual activity by the mother was only associated with upper-body strength and adult height, particularly of the female offspring. Several sexually dimorphic physical traits in offspring are associated with maternal activity levels during pregnancy. Prenatal testosterone is almost certainly involved. The associations could either reflect genetic influences (given that prenatal testosterone is highly heritable) or an effect of maternal testosterone being transferred to the developing fetus. More research is needed to assess the relative merit of these two possibilities."

(J Biosoc Sci. 2017 Apr 3:1-10. doi: 10.1017/S0021932017000074. [Epub ahead of print], MATERNAL ACTIVITY DURING PREGNANCY AND SEXUALLY DIMORPHIC TRAITS IN OFFSPRING. Ellis L, Eisenman R, Hoskin A)

Exposure of rat fetuses to phenobarbital lowers testosterone in plasma and brain for rest of life

"Exposure of rats to phenobarbital during late prenatal development decreased the concentration of testosterone in plasma and the brain during the late fetal, early postnatal, pubertal, and adult periods, By decreasing the production of testosterone in the brain during the period of sexual differentiation, phenobarbital may lead to sexual dysfunction in later life."

(Gupta, Chhanda, Sumner J. Yaffe, and Bernard H. Shapiro. "Prenatal exposure to phenobarbital permanently decreases testosterone and causes reproductive dysfunction." Science 216.4546 (1982): 640-642.)

2D:4D digit ratio is highly heritable

"PT (Prenatal Testosterone) levels are highly heritable".

(Blanchard, Alyson, and Luna C. Munoz Centifanti. "Callous-Unemotional Traits Moderate the Relation Between Prenatal Testosterone (2D: 4D) and Externalising Behaviours in Children." Child Psychiatry & Human Development (2016): 1-10., citing the study below)

"Overall, biometric model-fitting analyses indicated significant additive genetic and nonshared environmental influences on digit ratios. Findings suggest greater similarity between 2D:4D ratios in MZ (monozygotic) relative to DZ (dizygotic) twins that can be accounted for by genetic and nonshared environmental factors."

(Gobrogge, Kyle L., S. Marc Breedlove, and Kelly L. Klump. "Genetic and environmental influences on 2D: 4D finger length ratios: A study of monozygotic and dizygotic male and female twins." Archives of Sexual Behavior 37.1 (2008): 112-118.)

"Heritability was estimated to be approximately 66% for 2d:4d (...). [...] These results suggest a substantial genetic contribution to the determination of this hormonally related skeletal ratio in women, which could be more influential than the effects of common prenatal environmental factors." (study on female twins)

(Paul, Simon N., et al. "Heritability of the second to fourth digit ratio (2d: 4d): A twin study." Twin Research and Human Genetics 9.02 (2006): 215-219.)

"[P]arent‐offspring (0.15‐0.28, P < .001) and sibling correlations (0.13‐0.38, P < .009) were found to be significant. (...) thus indicating the existence of a clear familial aggregation of 2D:4D ratio variation in the Chuvashian pedigrees, which cannot be explained only by common environmental effects."

(Kalichman, Leonid, Valery Batsevich, and Eugene Kobyliansky. "Heritability estimation of 2D: 4D finger ratio in a Chuvashian population‐based sample." American Journal of Human Biology: e23212.)

Mother's exposure during pregnancy to stress and estrogens in food and chemicals may be linked to autism in children

"Mothers of children with ASD reported higher consumption of non-hormone free animal products, reported putting more endocrine mimicking chemicals on their bodies daily than control mothers, and reported having “very high” levels of stress during their pregnancies compared to control mothers. Furthermore, mothers of children with ASD reported a higher incidence of traumatic events during their pregnancies, specifically during the pregnancy with their child that received an ASD diagnosis, and a higher lifetime incidence of PTSD. Overall, there are significant differences related to maternal estrogen exposure between the pregnancies of mothers with and without a child with ASD. These results suggest that future research should investigate the possibility of a direct link between maternal
estrogen exposure and autism risk."

(Bunker, Sarah. "Maternal estrogen exposure may be linked to an increased risk of autism spectrum disorder." (2017).)

Maternal age and 2D:4D strong determinant of children's 2D:4D, periconceptual smoking not

"Age and maternal 2D:4D were strong determinants of the children’s 2D:4D, however, the mean 2D:4D did not differ among children whose mothers had smoked during the periconceptional period (i.e. 1 year before through the first trimester of pregnancy) compared with those who had not, irrespective of sex. In conclusion, we did not find an association between maternal periconceptional smoking and children’s 2D:4D, although the smoking prevalence was low."

(Velez, M. P., et al. "Is maternal periconceptional smoking associated with 2D: 4D digit ratio in their children?.")

2D:4D not influenced by prenatal exposure to famine

"The 2D:4D ratio is not significantly affected by prenatal exposure to famine."

(Stein, Aryeh D., Henry S. Kahn, and L. H. Lumey. "The 2D: 4D digit ratio is not a useful marker for prenatal famine exposure: Evidence from the Dutch hunger winter families study." American Journal of Human Biology 22.6 (2010): 801-806.)

Low digit ratios in early hominids and humans, except Australopithecus

"Pierolapithecus catalaunicus, Hispanopithecus laietanus and Ardipithecus ramidus have ratios consistent with polygynous extant species (low 2D:4D), whereas the ratio of Australopithecus afarensis is consistent with monogamous extant species (high 2D:4D). The early anatomically modern human Qafzeh 9 and Neanderthals have lower digit ratios than most contemporary human populations, indicating increased androgenization and possibly higher incidence of polygyny."

(Nelson, Emma, et al. "Digit ratios predict polygyny in early apes, Ardipithecus, Neanderthals and early modern humans but not in Australopithecus." Proceedings of the Royal Society of London B: Biological Sciences 278.1711 (2011): 1556-1563.)

"Australopithecus [...] were the first hominids to possess genes, known as the duplicated SRGAP2, which increased the length and ability of neurons in the brain.[2] One of the australopith species eventually became the Homo genus in Africa around two million years ago (e.g. Homo habilis), and eventually modern humans, H. sapiens sapiens."

(Wikipedia, Australopithecus, accessed 2017/26/05)

More maternal exposure to light toward end of 1st prenatal trimester predicts low 2D:4D in offspring

"Low 2D:4D (high prenatal testosterone) is found in Winter Births.
The effect is greatest for births in the first part of the 20th Century.
Day length towards the end of the 1st Trimester predicts 2D:4D
Our results support the solstititial-melatonin-testosterone hypothesis"

"Participants born in the late-Autumn and who experienced long days in the 2nd and 3rd prenatal months had low 2D:4D. The effects were strongest for early 20th Century births where photoperiods would be less disrupted by artificial light."

(Szwed, Anita, Magdalena Kosinska, and John T. Manning. "Digit ratio (2D: 4D) and month of birth: A link to the solstitial-melatonin-testosterone effect." Early Human Development 104 (2017): 23-26.)

2D:4D transmitted from mother to daughter but not mother/father to son

"Correlations between 2D:4D of parents and children, as well as between mothers and fathers were conducted. 2D:4D was also examined in relation to age and sex.
Results and conclusions:
(...) Within the dyads, there was a significant positive correlation between mother and daughter 2D:4D, but no significant correlation between mother and son ratios, nor between father and offspring ratios. The overall pattern of correlations (with emphasis on father-son dyads) was not supportive of Y-linkage. There was a positive correlation between 2D:4D and age in children, and a negative correlation between 2D:4D and age in adults, and no evidence of assortative mating. Our data are consistent with the notion of 2D:4D as a sexually-dimorphic, mildly age-sensitive, and transgenerationally-transmitted trait that is more likely to be X- than Y-linked."

(Richards, Gareth, Wynford Bellin, and William Davies. "Familial digit ratio (2D: 4D) associations in a general population sample from Wales." Early Human Development 112 (2017): 14-19.)

Prenatal androgen exposure may be related to maternal age and weight gain during pregnancy

"Multiparity ( = -0.28, p < 0.001), self-rated depression ( = 0.26, p < 0.001), and weight gain ( = 0.18, p < 0.05) were independent explanatory factors for the maternal total testosterone levels. Maternal age ( = -0.34, p < 0.001), weight gain ( = 0.19, p < 0.05), and amniotic fluid cortisol levels ( = 0.44, p < 0.001) were independent explanatory factors of amniotic fluid testosterone in female fetuses, explaining 64.3 % of the variability in amniotic fluid testosterone.
Wider implications of the findings: Young maternal age and excessive maternal weight gain may increase the prenatal androgen exposure of female fetuses. Further studies are needed to explore this finding."

(Kunovac-Kallak, Theodora, et al. "Maternal and female fetal testosterone levels are associated with maternal age and gestational weight gain." European Journal of Endocrinology (2017): EJE-17.)

Testosterone transmits poorly from maternal into fetal body (study on rats)

"In the present study it was found that doses of 10 and 25 mg testosterone propionate (TP) given to pregnant rats could induce the early-androgen syndrome. These doses caused delayed parturition with intra-uterine death and inhibition of lactation with early post-natal death of the young; these events could be avoided by Caesarian section and foster mothers.
In other experiments TP was injected directly into the foetuses or their amniotic sacs. A dose of 20 μg into the amniotic sac was without effect on subsequent ovarian activity and, with one exception, on the external genitalia of the young. Injection of 20 or 100 μg into the foetus caused both external masculinization and anovulatory ovaries.
The apparent low sensitivity of the foetus to TP injected into the pregnant doe seems to be due to poor transmission of the steroid from the maternal into the foetal body. The foetal external genitalia appear to be more sensitive to TP than the mechanism which sets the future gonadotrophic pattern of the brain-pituitary complex."

(Swanson, H. E., and JJ van der Werff ten Bosch. "THE» EARLY-ANDROGEN «SYNDROME; EFFECTS OF PRE-NATAL TESTOSTERONE PROPIONATE." Acta endocrinologica 50.3 (1965): 379-390.)

11 pesticides, fungicides and pharmaceuticals identified that suppress testosterone synthesis in fetal testes cell cultures; effect increased when mixed together

"Numerous chemicals have been identified as antiandrogens in experimental animals and cell-based systems and have been shown to induce androgen insufficiency by several modes of action: suppressing androgen synthesis, blocking the androgen receptor, or altering the signaling of local mediators such as prostaglandins. Prominent examples of anti-androgenic chemicals include certain phthalates used as plasticizers, pesticides, and mild analgesics (...).
[...]
Candidate chemicals were selected for inclusion in our multi-component mixtures if they met the following criteria: a) They had to be able to suppress testosterone synthesis in the FEGA, and b) they had to show dose–response curves with almost complete inhibition of testosterone synthesis. (...) Furthermore, all selected chemicals had to be of relevance to human exposures. (...) 11 compounds qualified for inclusion in our mixtures [namely Ketoconazole, BPA, Valproic acid, Clomiphene, Theophylline, BPS, Chlordecone, Imazalil, Bitertanol, Prochloraz, and Propiconazole].  (...) [Our work] uncovered several new chemicals whose capability of suppressing testosterone synthesis in humans was not previously recognized: the pesticide imazalil, (...); the pesticides propiconazole and bitertanol, (...) and the fungicide prochloraz and the selective estrogen receptor modulator clomifen (...), [t]heophylline, a caffeine metabolite and pharmaceutical used in the treatment of asthma (...) [,] the teratogenic drug valproic acid and the pesticide chlordecone.(...)
OBJECTIVES:
Our aim was to investigate whether chemicals can act together to disrupt androgen production in human fetal testis explants and to evaluate the importance of mixture effects when characterizing the hazard of individual chemicals.
RESULTS:
We evaluated mixtures composed of four and eight antiandrogens that contained the pharmaceuticals ketoconazole and theophylline and several previously untested chemicals, such as the pesticides imazalil and propiconazole. Mixtures of antiandrogens can suppress testosterone synthesis in human fetal testicular explants to an extent greater than that seen with individual chemicals. This revealed itself as a shift towards lower doses in the dose–response curves of individual antiandrogens that became more pronounced as the number of components increased from four to eight."

(Gaudriault, Pierre, et al. "Endocrine Disruption in Human Fetal Testis Explants by Individual and Combined Exposures to Selected Pharmaceuticals, Pesticides, and Environmental Pollutants." Environmental Health Perspectives 87004: 1.)

Meat-eating correlated with low 2D:4D ratio

"The key factor impacting offspring development (including prenatal androgen levels) appears to be diet, both in terms of shortage and excessive intake of certain food products. (...) One of the findings is that countries with high meat consumption present the so-called masculine digit ratio, while countries with plant-based diets - a feminine digit ratio."

(Modlinska, Klaudia, and Wojciech Pisula. "exploratory analysis of the Links among Food Consumption profiles, prenatal androgens, and selected Measures of Quality of Life." Frontiers in public health 4 (2016).)

Hyperplasia of adrenal glands causes embryonic exposure to abnormally high levels of androgens

"There are illnesses which affect the embryonic hormonal balance [...]. For example, hyperplasia of the adrenal glands consists of an abnormal development of these glands which then no longer secrete cortisol but which produce androgens instead. "The girls affected are often exposed to much higher levels of male hormones than normal during the course of their embryonic life. This results in a masculinisation of genital structures. Some are born with a fusion of the lips or a clitoris the size of a penis etc.", explains Jacques Balthazart."

(Balthazart, Jacques. "Sexual orientation in all its aspects.", Reflexions, le site de vulgarisation de l'Université de Liège, Université de Liège - http://reflexions.ulg.ac.be/ - 06 July 2017)

Sexual differentiation of mammal brain caused by hormonal, genetic and epigenetic factors

(Balthazart, Jacques. "Sexual orientation in all its aspects.", Reflexions, le site de vulgarisation de l'Université de Liège, Université de Liège - http://reflexions.ulg.ac.be/ - 06 July 2017)

Paracetamol during pregnancy mildly lowers fetal testosterone

"Gestational exposure to acetaminophen causes a mild reduction in fetal testosterone production in the rat and human fetal testis"

(Al-Shaikh, Turki M. "Histological Study on the Effects of Captopril, Paracetamol and Their Combination on Testis in Rats.")

Actual testosterone levels are correlated with ambient temperature and season (study on monkeys)

"Fecal T levels correlated with both rank and atmospheric temperature."

(Takeshita, Rafaela SC, et al. "Effect of castration on social behavior and hormones in male Japanese macaques (Macaca fuscata)." Physiology & Behavior (2017).)

Environmental factors are primary cause of testicular dysgenesis syndrome

"This review aimed to provide evidence verifying the reality of TDS (testicular dysgenesis syndrome) based on four key aspects: environmental endocrine-disrupting chemicals (EDCs), genetic factors, intrauterine growth disorders and lifestyle factors. (...) In conclusion, our study revealed that environmental exposures owing to modern lifestyles are primary factors involved in the associated trends of the syndrome, which are capable of affecting the adult endocrine system via direct means or through epigenetic mechanisms."

(Xing, Jian-Sheng, and Zhi-Ming Bai. "Is testicular dysgenesis syndrome a genetic, endocrine, or environmental disease, or an unexplained reproductive disorder?." Life Sciences (2017).)

People with Klinefelter syndrome have higher 2D:4D ratio

"Adult KS (Klinefelter syndrome) individuals were taller than their fathers and had shorter fingers than fathers and male controls. Compared with fathers, male controls and mothers, KS males had shorter fingers relative to height. With regard to 2D:4D, KS individuals had higher 2D:4D than fathers (right and left hands), male controls (right and left hands) and mothers (left hands). Among KS males older than 13 years there were 34 individuals currently prescribed testosterone and nine not prescribed. In comparison to the former, the latter individuals had higher right 2D:4D and higher right-left 2D:4D."

(Manning, John T., Liam P. Kilduff, and Robert Trivers. "Digit ratio (2D: 4D) in Klinefelter's syndrome." Andrology 1.1 (2013): 94-99.)

Phtalates (plasticizers) reduce testosterone to female levels in male rat offspring

"Phthalate esters (PE) such as DEHP are high production volume plasticizers used in vinyl floors, food wraps, cosmetics, medical products, and toys. In spite of their widespread and long-term use, most PE have not been adequately tested for transgenerational reproductive toxicity. (...) Maternal DEHP treatment at 750 mg/kg/day from gestational day (GD) 14 to postnatal day (PND) 3 caused a reduction in T production, and reduced testicular and whole-body T levels in fetal and neonatal male rats from GD 17 to PND 2. As a consequence, anogenital distance (AGD) on PND 2 was reduced by 36% in exposed male, but not female, offspring. By GD 20, DEHP treatment also reduced testis weight.(...) These data indicate that DEHP disrupts male rat sexual differentiation by reducing T to female levels in the fetal male rat during a critical stage of reproductive tract differentiation."

(Parks, Louise G., et al. "The plasticizer diethylhexyl phthalate induces malformations by decreasing fetal testosterone synthesis during sexual differentiation in the male rat." Toxicological sciences 58.2 (2000): 339-349.)

Lavender and tea-tree oil have estrogenic and anti-androgenic properties

"Researchers at the NIEHS, including Kenneth Korach, Ph.D., a co-investigator for the new study, previously found laboratory evidence that lavender and tea tree oil have estrogenic (estrogen-like) properties and anti-androgenic (testosterone inhibiting-like) activities, meaning they compete or hinder the hormones that control male characteristics, which could affect puberty and growth."

(The Endocrine Society. "Chemicals in lavender and tea tree oil appear to be hormone disruptors." ScienceDaily. ScienceDaily, 18 March 2018. )

Side-product of Brazilian bio-fuel production is very potent antiandrogenic, and exposure is ubiquitous in Brazil

"Diisopentyl phthalate (DiPeP) exposure in Brazil seems ubiquitous. DiPeP is produced from iso-amyl alcohol, a side product of Brazilian bio-fuel. (...) This unexpected and ubiquitous DiPeP exposure indicates to unique DiPeP exposure sources in Brazil. These exposures spark considerable concern because DiPeP is one of the most potent antiandrogenic phthalates."

(Souza, Michele Bertoncello, Marcella Tapias Passoni, Claudia Pälmke, Katlyn Barp Meyer, Amanda Caroline Venturelli, Giulia Araújo, Bruno Sanches de Castilhos et al. "Unexpected, ubiquitous exposure of pregnant Brazilian women to diisopentyl phthalate, one of the most potent antiandrogenic phthalates." Environment International 119 (2018): 447-454.)

Maternal 2D:4D correlates with child's 2D:4D; Time-to-pregancy, infertility and Assisted Reproductive Technologies have no influence on child's 2D:4D

"We aimed to investigate the association between TTP (Time To Pregnancy, the number of menstrual cycles required to conceive), infertility, or use of Assisted Reproductive Technologies (ART) and offspring 2D:4D. (...) The mean 2D:4D (of the offspring) did not differ according to TTP, infertility, or use of ART. The only factors associated with the child's 2D:4D were the child's age and maternal 2D:4D."

(Shere, Mahvash, Tye E. Arbuckle, Patricia Monnier, William Fraser, and Maria P. Velez. "Time‐to‐pregnancy and offspring finger‐length ratio (2D: 4D)." American Journal of Human Biology.)

Factors influencing maternal androgen concentrations: with age maternal testosterone concentrations decrease, with BMI they increase, and black women have 18-30% higher androgen concentrations

"Objectives To identify factors predicting maternal sex steroid hormone concentrations in early pregnancy.
Methods The Infant Development and the Environment Study recruited healthy pregnant women from academic medical centers in four US cities. Gold standard liquid chromatography–tandem mass spectrometry was used to measure maternal sex steroids concentrations (total testosterone [TT], free testosterone [FT], estrone [E1], estradiol [E2], and estriol [E3] concentrations) in serum samples from 548 women carrying singletons (median = 11.7 weeks gestation). Women completed questionnaires on demographic and lifestyle characteristics.
Results In multivariable linear regression analyses, hormone concentrations varied in relation to maternal age, body mass index (BMI), race, and parity. Older mothers had significantly lower levels of most hormones; for every year increase in maternal age, there was a 1–2% decrease in E1, E2, TT, and FT. By contrast, each unit increase in maternal BMI was associated 1–2% lower estrogen (E1, E2, E3) levels, but 1–2% higher androgen (TT, FT) concentrations. Hormone concentrations were 4–18% lower among parous women, and for each year elapsed since last birth, TT and FT were 1–2% higher (no difference in estrogens). Androgen concentrations were 18–30% higher among Black women compared to women of other races. Fetal sex, maternal stress, and lifestyle factors (including alcohol and tobacco use) were not related to maternal steroid concentrations.
Conclusions for Practice Maternal demographic factors predict sex steroid hormone concentrations during pregnancy, which is important given increasing evidence that the prenatal endocrine environment shapes future risk of chronic disease for both mother and offspring."

(Barrett, Emily S., Omar Mbowe, Sally W. Thurston, Samantha Butts, Christina Wang, Ruby Nguyen, Nicole Bush et al. "Predictors of Steroid Hormone Concentrations in Early Pregnancy: Results from a Multi-Center Cohort." Maternal and Child Health Journal (2019): 1-11.)

People with congenital GH/IGF-1 deficiency have higher 2D:4D digit ratio

"The analyses of our results revealed that patients with congenital GH/IGF-1 deficiency [Growth Hormone/Insuline-like Growth Factor] show an identical 2D:4D ratio for both sexes, but a higher (more feminine) ratio than the normal population. These findings may be explained by a higher estrogen effect resulting from the absence of a functional GH-IGF-1 axis prenatally."

(Uchitel, Y., R. Kauli, O. Konen, P. Lilos, and Z. Laron. "The second to fourth digit ratio in patients with congenital IGF-1 Deficiency." Anthropologischer Anzeiger; Bericht uber die biologisch-anthropologische Literatur (2019).)

Higher number of older brothers and number of children in the family increases 2D:4D in boys but decreases 2D:4D in girls

"Birth order, number of siblings (except for younger sisters), and sex ratio in siblings were significantly related to the 2D:4D ratio on the right, left, or both hands. At the same time, the relationships were opposite in males and females for birth order, number of older brothers and number of children in the family––increasing values in these variables increased digit ratio in males but decreased digit ratio in females."

(Králík, Miroslav, Adela Hupková, Tomáš Zeman, Martin Hložek, Lukáš Hlaváček, Marie Slováčková, Martin Čuta, and Petra Urbanová. "Family effects on the digit ratio (2D: 4D): The role of the interbirth interval." American Journal of Human Biology (2019): e23260.)

Inbreeding associated with lower digit ratios in offspring

"We determined that inbreeding was related to a reduction in the digit ratios regardless of the side of the hand and the sex of the infant, showing a consistent tendency to appear more masculine. Interaction between marriage type*sex was highly significant.
Conclusions
Parental consanguinity appears to cause fetal masculinization of digit ratios by increasing fetal stress among Turkish newborns."

(Ertuğrul, Berna, and Barış Özener. "Relationship between parental consanguinity and digit ratios (2D: 4D) among Turkish newborns in Sivas." American Journal of Human Biology (2019).)

In rats, maternal exposure to ibuprofen has "low testosterone" effects on offspring

"Males [(male offspring of mothers)] exposed to ibuprofen had a decrease in body weight and anogenital distance, as well as a delay in the ages of testicular descent and preputial separation. In adulthood, there was a decrease in the Leydig cells nuclei volume, testosterone levels and percentage of normal sperm morphology. All animals exposed to ibuprofen presented male copulatory behavior, however, in the presence of another male, they also presented a female-typical behavior. Maternal exposure to ibuprofen during the sensitive windows of brain development adversely impacted the reproductive parameters of male rats, suggesting an incomplete masculinization of the hypothalamus."

(da Silva Balin, P., Jorge, B. C., Leite, A. R. R., Borges, C. S., Oba, E., Silva, E. J. R., ... & Arena, A. C. (2020). Maternal exposure to ibuprofen can affect the programming of the hypothalamus of the male offspring. Regulatory Toxicology and Pharmacology, 104576.)

Phtalates and BPA exposure increases 2D:4D in boys, not girls; mediated by estrogen receptor ESR1 polymorphism type

"Phthalates and bisphenol A (BPA) are estrogenic endocrine disruptors. Polymorphisms in the gene encoding estrogen receptor 1 (ESR1) may contribute to the ratio of the lengths of the second and fourth digits (2D:4D), which is considered an index of prenatal exposure to sex hormones. Thus, we investigated whether ESR1 polymorphisms modify the effects of prenatal exposure to phthalates and BPA on 2D:4D in a birth cohort. (...)
Boys with the AG/GG genotype at rs2077647 in the group exposed to high levels of mono(2-ethylhexyl) phthalate (MEHP) or Σ Di(2-ethylhexyl) phthalate (DEHP) showed feminized 2D:4D compared with boys with the AA genotype at rs2077647 who had low exposure to MEHP or ΣDEHP (...). No significant differences were found among girls. There were no associations between mean 2D:4D and metabolites other than MEHP or BPA. These data suggest that ESR1 polymorphisms modify the effects of prenatal exposure to DEHP on mean 2D:4D among boys."

(Nishimura, Y., Moriya, K., Kobayashi, S., Araki, A., Sata, F., Mitsui, T., ... & Nakamura, M. (2020). Association of exposure to prenatal phthalate esters and bisphenol A and polymorphisms in the ESR1 gene with the second to fourth digit ratio in school-aged children: data from the Hokkaido Study. Steroids, 108637.)

 

In rats, developmental vitamin D defincieny increases testosterone levels in male foetuses brains

"We show that DVD-deficiency (Developmental Vitamin D) increases testosterone levels in maternal blood. We also show elevated levels of testosterone and androstenedione in the amniotic fluid of female but not male DVD-deficient foetuses. Testosterone levels were also elevated in DVD-deficient male brains."

(Ali, A. A., Cui, X., Pertile, R. A. N., Li, X., Medley, G., Alexander, S. A., ... & Eyles, D. W. (2020). Developmental vitamin D deficiency increases foetal exposure to testosterone. Molecular Autism, 11(1), 1-13.)

Bisphenol analogues associated with higher 2D:4D

"Prenatal exposure to BPA alternatives was associated with offspring’s higher 2D:4D."

(Wang, Z., Zhou, Y., Liang, H., Miao, M., Chen, Y., Zhang, X., ... & Yuan, W. (2021). Prenatal exposure to bisphenol analogues and digit ratio in children at ages 4 and 6 years: A birth cohort study. Environmental Pollution, 278, 116820.)