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.)

Studies indirectly related to (prenatal) testosterone

Dominant (high testosterone?) personalities get paid more

"Dominant, assertive women, who clearly express their expectations and do not retreat from their demands, are compensated better than their more accommodating female peers. According to the researchers, the same goes for dominant men versus their more conciliatory male counterparts [...]."

('Nice' women earn less than their more assertive counterparts, ScienceDaily, nov.21st 2016)

High testosterone type men more likely to have psychological problems

"Men who see themselves as playboys or as having power over women are more likely to have psychological problems than men who conform less to traditionally masculine norms [...].
Specifically, they focused on three broad types of mental health outcomes: negative mental health (e.g., depression), positive mental health (e.g., life satisfaction), and psychological help seeking (e.g., seeking counseling services). [...]
While overall, conforming to masculine norms was associated with negative mental health outcomes in subjects, the researchers found the association to be most consistent for these three norms -- self-reliance, pursuit of playboy behavior, and power over women."

(Sexism may be harmful to men's mental health, ScienceDaily, nov.21 2016)

Trauma changes brain differently in boys than in girls

"Within [the insula's anterior circular sulcus], boys with PTSD (Post-Traumatic Stress Disorder) symptoms demonstrated larger volume and surface area than control boys, while girls with PTSD symptoms demonstrated smaller volume and surface area than control girls."

(Klabunde, Megan, et al. "The moderating effects of sex on insula subdivision structure in youth with posttraumatic stress symptoms." Depression and Anxiety (2016).)

Dominance (high testosterone?) related to psychopathology, mania and narcissism

"Extensive research suggests that externalizing disorders, mania-proneness, and narcissistic traits are related to heightened dominance motivation and behaviors. Mania and narcissistic traits also appear related to inflated self-perceptions of power. Anxiety and depression are related to subordination and submissiveness, as well as a desire to avoid subordination."

(Johnson, Sheri L., Liane J. Leedom, and Luma Muhtadie. "The dominance behavioral system and psychopathology: evidence from self-report, observational, and biological studies." Psychological bulletin 138.4 (2012): 692.)

"[N]arcissism is positively associated with dominance (Bradlee & Emmons, 1992; Emmons, 1987; Raskin & Terry, 1988)."

(Grijalva, Emily, and Peter D. Harms. "Narcissism: An integrative synthesis and dominance complementarity model." The Academy of Management Perspectives 28.2 (2014): 108-127.)

Higher rates of homosexuality, bisexuality and asexuality among people with autism

"In the group with ASD (Autism Spectrum Disorder), 69.7% of the sample reported being non-heterosexual, while in the TD (Typically Developing) group, 30.3% reported being non-heterosexual. The group with ASD reported higher rates of homosexuality, bisexuality and asexuality, but lower rates of heterosexuality. The results support the impression that non-heterosexuality is more prevalent in the autistic population."

(George, R., and M. A. Stokes. "Sexual Orientation in Autism Spectrum Disorder." Autism Research.)

Estrogens from contraceptive pill implicated in agressive behavior

"Ethinylestradiol (EE2) is a widely produced, powerful synthetic estrogen that is widespread in the environment mainly because is a component of the contraceptive pill. In addition, fetuses may be exposed to EE2 when pregnancy is undetected during contraceptive treatment. To understand whether exposure to EE2 during gestation or lactation affects social play, we exposed 72 female Sprague-Dawley rats to EE2 or vehicle either during gestation (gestation day (GD) 5 through GD 20) or during lactation (from postnatal day (PND) 1 through PND 21). Two doses of EE2 were used to treat the dams: a lower dose in the range of possible environmental exposure (4 ng/kg/day) and a higher dose equivalent to that received during contraceptive treatment (400 ng/kg/day). Behavioral testing was carried out between PND 40 and 45. A principal component analysis of frequencies of behavioral items observed during play sessions identified three main components: defensive-like play, aggressive-like play, and exploration. Aggressive-like play was significantly increased by both doses of EE2, and the gestational administration was in general more effective than the lactational one. Defensive-like play and exploration were not significantly affected by treatment. This research showed that low and very low doses of EE2 that mimic clinical or environmental exposure during development can affect important aspects of social behavior even during restricted time windows."

(Zaccaroni, Marco, et al. "Developmental Exposure to Low Levels of Ethinylestradiol Affects Play Behavior in Juvenile Female Rats." Neurotoxicity Research (2017): 1-11.)

Endocrine disrupting chemicals are devastating the biosphere

(Translation from Portuguese to English by Google Translate)

"Introduction to the theme: With the unraveling of the various technological revolutions (industrial revolution, green revolution, among others), from generation to generation, the molecular pattern to which we are exposed on a daily basis has changed. This change culminated in significant impacts on our biology and, along with it, on our health and disease processes. Unrestricted to us, we are slowly altering the chemistry of the biosphere, bringing devastating consequences to the myriad forms of life on earth - as Rachel Carson brilliantly stated, "in nature, there is nothing alone."
Theoretical course: [...] [A] number of molecules with chemical similarity to female hormones can promote endocrine disruption, blocking [sexual] differentiation at certain points. Among the molecules, many of them are fruits of modern chemical revolutions, such as phthalates (widely present in plastics and plasticizers), pesticides, herbicides, some drugs, smoking, and other household products that have become our market list. (...)
Conclusion: At the dawn of technological revolutions, a problem arises not only of public health, but also of the environment: we are destroying a biological equilibrium slowly shaped by tens of thousands of years. We are stealing the future not only of our children, but of whole specimens. Such a problem permeates the political bed. And with chronic public disregard for science, fewer and fewer people are able to question this type of problem and call for change and improvement. This problem plagues our future and, as a true science, must be debated and disseminated."

(Müller, Juliane Centeno, et al. "A DISTÂNCIA ANOGENITAL DOS RECÉM-NASCIDOS E SUA PROFUNDA RELAÇÃO COM A BIOSFERA.")

Engineering culture discriminates empathizing brains

"This paper reviews relevant research on how systemizing-empathizing (S-E) theory applies to engineering education and examines current research on the reasons behind the dearth of females in engineering, finding that the contemporary engineering culture in college is also characterized by subtle forms of discrimination that systematically direct women away from engineering."

(Fertig, Jan, and Subha Kumpaty. "Gender Issues in Engineering Education: What Systemizing and Empathizing Have to Do With It." ASME 2017 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2017.)

Male brains perceive themselves as more intelligent than female brains, even if they score the same grades

A first-of-its-kind study shows that in the college biology classroom, men perceive themselves as smarter, even when compared to women whose grades prove they are just as smart. (...)
Katelyn Cooper, a doctoral student in the Arizona State University School of Life Sciences and lead author of the study, has talked with hundreds of students as an academic advisor and those conversations led to this project.
"I would ask students about how their classes were going and I noticed a trend," shared Cooper. "Over and over again, women would tell me that they were afraid that other students thought that they were 'stupid.' I never heard this from the men in those same biology classes, so I wanted to study it."

(Arizona State University. "Who's smarter in the classroom -- men or women? New study shows it's all about perception." ScienceDaily. ScienceDaily, 4 April 2018.)

Women (in general) experience pleasure when cooperating, men (in general) when they are cheating

"What we're going to see later is something very interesting: people playing [the Prisoner's Dilemma game] inside a brain scanner, [while the researchers are] looking into a part of the brain that has a lot to do with pleasure, and what you see is [that] some individuals activate that part of the brain when they've successfully stabbed the other guy in the back, some individuals activate it when they have both cooperated, and there's a big gender difference as to which circumstance. So you just guess which one is going on there (...)."

(March 31, 2010, Stanford professor Robert Sapolsky lectures on the biology of behavioral evolution, Youtube, "2. Behavioral Evolution", 44:36 - 45:04)

Men willing to punish more than women to get ahead

'Chapman University has published research measuring gender differences in cooperation and punishment behavior. Results showed that men punish more than women, men obtain higher rank, and punishment by males decreases payoffs for both sexes. Furthermore, men are willing to punish people who have done nothing wrong, except cooperate to the fullest extent possible. 

Results suggest that status-seeking men are willing to impose enormous costs on others and destroy their group to move up in the hierarchy. According to the study, men may punish more than women for two reasons: First, punishment is often viewed as similar to physical conflict. Men are known to favor physical punishment of unfair behavior. Men are also less cooperative and less generous compared with their female counterparts.
Second, status affects cooperative behavior and women may feel differently about status and rank. If so, punishment may be a tool used by certain individuals to advance in rank. For example, explicit rank-based incentives caused men to punish at roughly twice the rate of women. 

"Outside the laboratory, high-powered punishment and rank-based reward may be the norm," said Terence Burnham, Ph.D, associate professor in Chapman University's Argyros School of Business and Economics, and sole author of this study.'

(Chapman University. "Men willing to punish more than women to get ahead: New research evaluates gender differences in cooperation." ScienceDaily. ScienceDaily, 16 April 2018. )

There are much more severely antisocial males than females

"Males greatly outnumber females in the most severe forms of antisocial behavior."

(Eme, Robert. "Sex differences in temperament: A partial explanation for the sex difference in the prevalence of serious antisocial behaviors." Aggression and Violent Behavior (2018).

Men are more risk taking, overconfident, more willing to hurt others than women

Considerable evidence that women are significantly more averse to taking risks

Why?
▪ Emotions: Women report more nervousness and fear in anticipation of negative outcomes
▪ Overconfidence: Men being more overconfident in their success in uncertain situations
▪ Perception of risk: Men are more likely to see a risky situation as a challenge while females women risky situations as threats
(citing Croson, Rachel, and Uri Gneezy. 2009. "Gender Differences in Preferences." Journal of Economic Literature, 47 (2): 448-74, DOI: 10.1257/jel.47.2.448)
(...)

Why do women compete less?

▪ Differences in performance: Not the case in arithmetic
▪ Differences in beliefs: Men are overconfident
▪ Preferences for risk: Women are more risk averse
▪ Differences in altruism: Women do not want to hurt others
▪ Aversion to competition: Women dislike performing in competitions
(...)

Evidence of overconfidence, particularly by men

Men:
% expecting to be 1st 75%
Of those expecting to be 1st, 27% were actually first
Women
% expecting to be 1st 43%
Of those expecting to be 1st, 47% were actually first
(citing Niederle, Muriel, and Lise Vesterlund. "Do women shy away from competition? Do men compete too much?." The Quarterly Journal of Economics 122.3 (2007): 1067-1101.)
(...)

Women avoid performing in competitive environments

Replicated many times
Why a gap in tournament entry?
▪ Beliefs: gap weakens in tasks where women are expected to perform better (e.g. verbal tasks) and when feedback about relative performance is given
▪ “Culture”: gap weakens when competition is for teams and not individuals, in matrilineal societies, among young children, and for girls who attend same-sex schools
(citing Niederle, Muriel, and Lise Vesterlund. "Gender and competition." Annu. Rev. Econ. 3.1 (2011): 601-630.)

(EXPERIMENTAL ECONOMICS - GENDER, presentation by Ernesto reuben, associate professor in the Social Science Division, New York University, Abu Dhabi)

Substantial differences between female and male brains in mice

Scientists studying the locus coeruleus brain structure in mice unexpectedly found substantial differences in the molecular structures of this part of the brain between male and female mice.

(Cell Press. "Mice brain structure linked with sex-based differences in anxiety behavior." ScienceDaily. ScienceDaily, 22 May 2018. )

Teenage girls are more impacted by sleepiness than teen boys are

"What was most surprising is the fact that teenage girls reported a higher degree of interference of daytime sleepiness than teenage boys on multiple aspects of their school and personal activities," said co-author Pascale Gaudreault, who is completing her doctoral degree in clinical neuropsychology (...)."

(American Academy of Sleep Medicine. "Teenage girls are more impacted by sleepiness than teen boys are: Sleep disturbances in girls associated with more difficulties staying awake in and out of school." ScienceDaily. ScienceDaily, 6 June 2018.)

Gender is not purely a social construct

"We suggest, however, that the evidence for gender being purely a social construct is limited and neither necessary nor accepted as such within andrology. (...) Many problems in men’s health that have been assumed to be the products of enculturation have in fact a biological basis."

(Tsang, Vivian WL, and Richard J. Wassersug. "Men’s Health Research versus Andrology—Defining the Division and Closing the Divide."(PDF download) Journal of Men's Health 14.3 (2018): e20-e32.)

Study finds less corruption in countries where more women are in government

"In a cross-country analysis of over 125 countries, this study finds that corruption is lower in countries where a greater share of parliamentarians are women. The study further finds that women's representation in local politics is important too -- the likelihood of having to bribe is lower in regions with a greater representation of women in local-level politics in Europe."

(Virginia Tech. "Study finds less corruption in countries where more women are in government." ScienceDaily. ScienceDaily, 15 June 2018.)

The rupture of the masculinity-male and femininity-female associations

"Together, these findings suggest that gender stereotypes and self-attributions are changing in adolescents. Furthermore, our results can be considered to be an empirical confirmation of the rupture of the masculinity-male and femininity-female associations."

(Martínez-Marín, María Dolores, and Carmen Martínez. "Negative and Positive Attributes of Gender Stereotypes and Gender Self-Attributions: A Study with Spanish Adolescents." Child Indicators Research (2018): 1-21.)

Gender differences in psychopathology are quantitavie rather than qualitative, and related to brain masculinization/defeminization and acute testosterone levels

"Subtle gender differences in cognition arise due to the action of sex hormones during brain development, and are shaped by social influences [1-3]. Sex hormones also have immediate and reversible effects on cognition [4-6]. Women and men differ in specific measures of visuospatial and verbal ability, memory, and processing of emotional information [7,8].
The sexes also differ with respect to vulnerability to specific classes of psychopathology. For example, depressive and anxiety disorders are more prevalent in women, while autism, attention deficit hyperactivity disorder (ADHD) and Tourette Syndrome (TS) are more frequently observed in men [2,3]. Gender differences in psychopathology are most certainly a result of interactions between environmental factors and brain sexual dimorphisms. However, neuropsychiatric disorders also are often strongly influenced by genetic factors [9-11].
(...)
In sum, the present results point to complex interactions between gender and familial factors in determining premorbid characteristics, age of first psychotic episode, symptom severity, social functioning, and ultimate diagnosis. We propose that these gender differences may be related to quantitative variation in masculinization/defeminization of brain and cognition, as well as from acute effects of circulating estrogens and testosterone."

(Castro, Lina Díaz, Kurt Leroy Hoffman, Maria de Lourdes Nieto-García, Miriam Arroyo Belmonte, Hector Cabello-Rangel, Miguel Ángel Herrera-Estrella, Mariana Chávez-Cervantes, and Carlos Luis Aviña-Cervantes. "Gender Differences within the Psychosis Spectrum." Neuropsychiatry (London) 8, no. 4 (2018): 1186-1195.)

Ano-genital distance reflects prenatal androgen levels during early gestation, but not thereafter

"Our results suggest that AGD (Ano-Genital Distance) in humans, like animals, is fixed in early gestation (likely during the hypothesized MPW [masculinization programming window]) and is unaffected by androgen levels thereafter. Thus, AGD can serve as a biomarker of in utero androgen action during early gestation (likely 8–14 weeks) in humans."

(Jain, Viral G., Vaibhav Goyal, Vikas Chowdhary, Namita Swarup, Ravinder J. Singh, Arbinder Singal, and Prem Shekhawat. "Anogenital distance is determined during early gestation in humans." Human Reproduction (2018).)

Women more concerned about 'exploiting' social ties than men

"A new study reveals that it is not only exclusion by men, but also self-imposed barriers including hesitation and gendered modesty that prevent women from networking as effectively as their male counterparts. The research revealed that women's tendencies to harbor moral concerns about 'exploiting' social ties causes them to under-benefit from networking activities."

(SAGE. "Women build less effective professional networks than men as they underestimate self-worth." ScienceDaily. ScienceDaily, 15 November 2018.)

Men remember pain differently (causing them to be more sensitive to repeated pain) than women

"We believe that the mice and the men were anticipating [pain trigger] and, for the males, the stress of that anticipation caused greater pain sensitivity," says Mogil. "There was some reason to expect that we would see increased sensitivity to pain on the second day, but there was no reason to expect it would be specific to males. That came as a complete surprise."

(McGill University. "Men and women remember pain differently: Strength of finding confirmed by replication of results in mice and men." ScienceDaily. ScienceDaily, 10 January 2019. )

Facial masculinity not asscoiated with immunocompetence

"[W]e measured immunocompetence via heterozygosity at the major histocompatibility complex (MHC), a widely-used genetic marker of immunity. We show that, while height is positively correlated with MHC heterozygosity, facial masculinity is not. Thus, facial masculinity does not reflect immunocompetence measured by MHC heterozygosity in humans."

(Zaidi, Arslan A., Julie D. White, Brooke C. Mattern, Corey R. Liebowitz, David A. Puts, Peter Claes, and Mark D. Shriver. "Facial masculinity does not appear to be a condition-dependent male ornament and does not reflect MHC heterozygosity in humans." Proceedings of the National Academy of Sciences (2019): 201808659.)

Entrepreneurs like competition for the competition, not for winning or for personal development

"Referring to Isreal M. Kirzner (1973) and Schumpeter (1934), who emphasized the competitive nature of entrepreneurship, this study investigates whether potential and revealed entrepreneurs are more likely to seek competition than non-entrepreneurs. We provide a conceptual framework that links entrepreneurship to three facets of individual competitiveness drawn from economic, entrepreneurship, and psychological research: a desire to win, striving for personal development, and an enjoyment of competition. Following economic research linking competitive behavior in experiments to career choices, we conduct a lab-in-the-field study and demonstrate that entrepreneurs are more likely to enter competitions than non-entrepreneurs. Accounting for individual desires to win and mastery-related achievement motivations, our results indicate that entrepreneurs tend to enter competition for the sake of competition itself rather than for the prospect of winning it or personal development. Our results suggest that enjoyment of competition might be an additional factor driving entrepreneurs’ market entry decisions beyond well-known factors like overconfidence and risk-taking."

(Urbig, Diemo, Werner Bönte, Vivien D. Procher, and Sandro Lombardo. "Entrepreneurs embrace competition: evidence from a lab-in-the-field study." Small Business Economics (2019): 1-22.)

Females with specific genotype resemble males in Autism Quotient-Imagination subscale

"The extreme male brain theory of autism posits that its male bias is mediated by exaggeration of male-biased sex differences inthe expression of autism-associated traits found in typical populations. The theory is supported by extensive phenotypicevidence, but no genes have yet been described with properties that fit its predictions. The autophagy-associated gene AMBRA1 represents one of the top genome-wide “hits” in recent GWAS studies of schizophrenia, shows sex-differential expression, and has been linked with autism risk and traits in humans and mice, especially or exclusively among females. Wegenotyped the AMBRA1 autism-risk SNP in a population of typical humans who were scored for the dimensional expressionof autistic and schizotypal traits. Females, but not males, homozygous for the GG genotype showed a significant increase inscore for the single trait, the Autism Quotient-Imagination subscale, that exhibits a strong, significant male bias in typicalpopulations. As such, females with this genotype resembled males for this highly sexually dimorphic, autism-associated phenotype. These findings support the extreme male brain hypothesis and indicate that sex-specific genetic effects canmediate aspects of risk for autism."

(CRESPI, Bernard, et al. AMBRA1, Autophagy, and the Extreme Male Brain Theory of Autism. Autism Research and Treatment, 2019, 2019.)

Higher adult androgen exposure associated with higher handgrip strength and less emotionality

"2D:4D ratios and handgrip strength tests are two protocols that indirectly measure androgen exposure prenatally and in adulthood respectively. (...) Athletes who scored higher on the handgrip strength test showed lower levels of emotionality."

(BOURKE, Colm. The Characteristics of Successful Student Athletes. 2019. PhD Thesis. University of Louisiana at Monroe.)

Unethical behavior associated with higher facial width-to-height ratio in male and female athletes

"Past research has emphasized the role of facial structures in predicting social behavior. In particular the facial width-to-height ratio (fWHR) was found to be a reliable predictor for antisocial and unethical behavior. The current study was aimed at examining this association in the field of sports: FWHRs of 146 doping sanctioned athletes in athletics (37 male/38 female) and weightlifting (44 male/27 female) were compared to the fWHRs of randomly chosen non-doping sanctioned athletes of the Top Ten at the World Championship 2017 and Olympic Games 2016 in both sports (146 athletes). The results showed that doping sanctioned athletes due to the use of anabolic steroids had larger fWHRs than non-doping sanctioned athletes. However, doping sanctioned athletes due to other doping rule violations than the use of anabolic steroids, did not show this effect. The study provides empirical evidence for the relation between fWHR and unethical behavior in a real-world setting and contributes to the discussion about fWHR’s biological origin, emphasizing the role of anabolic steroids. A mutual interaction between fWHR and doping behavior is discussed, at which a larger fWHR might signify a higher tendency to behave unethically, whereas the consequential intake of anabolic steroids might also shape individuals’ faces."

(Krenn, B., and C. Buehler. "Facial features and unethical behavior-Doped athletes show higher facial width-to-height ratios than non-doping sanctioned athletes." PloS one 14.10 (2019): e0224472-e0224472.)

Cognitive rigidity associated with extremist attitudes

"Research into the roots of ideological extremism has traditionally focused on the social, economic, and demographic factors that make people vulnerable to adopting hostile attitudes toward outgroups. However, there is insufficient empirical work on individual differences in implicit cognition and information processing styles that amplify an individual’s susceptibility to endorsing violence to protect an ideological cause or group. Here we present original evidence that objectively assessed cognitive inflexibility predicts extremist attitudes, including a willingness to harm others, and sacrifice one’s life for the group. Across two samples (N = 1,047) from the United Kingdom and United States, structural equation models demonstrated that cognitive inflexibility predicted endorsement of violence to protect the national ingroup, which in turn predicted a willingness to die for the group. These statistical models accounted for an average of 31.4% of the variance in willingness to die for the group, after accounting for demographic variables. Furthermore, cognitive inflexibility was related to greater confidence in the decision to sacrifice one’s life in an ingroup trolley problem scenario. Analysis of participants’ performance on the cognitive tasks revealed that cognitive rigidity – distinctly from other aspects of cognition – was specifically implicated as a cognitive antecedent of extremist attitudes. Implications for the study of radicalization and identity fusion through a neurocognitive lens are discussed."

(Zmigrod, Leor, Peter Jason Rentfrow, and Trevor W. Robbins. "Cognitive inflexibility predicts extremist attitudes." Frontiers in psychology 10 (2019): 989.)

More feminine gender identity associated with more ethical behavior (also among men)

"This study shows that gender identity affects ethical intentions. We investigate the intention to purchase ethical products through a survey study among young consumers in Italy. Measures of planned behavior, internal ethics, self‐identity, and moral harm, together with proxies for individual gender identities of femininity and masculinity are included in our model of intention to purchase ethically. Results show that femininity significantly increases ethical intent, whereas masculinity has an opposite effect. These findings are robust to gender. In fact, the relations of femininity and masculinity on the intention to consume ethical products hold when the subsamples of males and females are considered."

(Pinna, Mariella. "Do gender identities of femininity and masculinity affect the intention to buy ethical products?." Psychology & Marketing.)

Autism associated with facial femininity in men, and facial masculinity in women

"In the present study, we conducted two experiments to examine facial masculinity/femininity in 151 neurotypical adults selected for either low, mid‐range, or high levels of autistic traits. In the first experiment, their three‐dimensional facial photographs were subjectively rated by 41 raters for masculinity/femininity and were objectively analysed. In the second experiment, we generated 6‐face composite images, which were rated by another 36 raters. Across both experiments, findings were consistent for ratings of photographs and composite images. For females, a linear relationship was observed where femininity ratings decreased as a function of higher levels of autistic traits. For males, we found a U‐shaped function where males with mid‐range levels of traits were rated lowest on masculinity. Objective facial analyses revealed that higher levels of autistic traits were associated with less feminine facial structures in females and less masculine structures in males. These results suggest sex‐specific relationships between autistic traits and facial masculinity/femininity."

(Tan, Diana Weiting, Murray T. Maybery, Louise Ewing, Jia‐Xin Tay, Peter R. Eastwood, and Andrew JO Whitehouse. "Sex‐specific variation in facial masculinity/femininity associated with autistic traits in the general population." British Journal of Psychology (2019).)

Women find artificially masculinized men more attractive

(Clarkson, Tessa R., Morgan J. Sidari, Rosanna Sains, Meredith Alexander, Melissa Harrison, Valeriya Mefodeva, Samuel Pearson, Anthony J. Lee, and Barnaby JW Dixson. "A multivariate analysis of women's mating strategies and sexual selection on men's facial morphology." Royal Society Open Science 7, no. 1 (2020): 191209.)

Women slower on mental rotation test, but with equal results; men and women approach spatial tasks differently

"Dr Toth sums up the results: "So males are better than females? Well no, actually. Our study found that there is no male advantage in mental rotation abilities. By lengthening the time allowed to complete the test, the male performance advantage diminished entirely suggesting that the so-called sex difference in mental rotation is simply not there or may be explained by other factors."

The research published in Nature Scientific Reports also found for the first time that both males and females frequently employed different gaze strategies during the cognitive tests to get to the correct answer. In other words, men and women approach the task in a different way to get the same result.

The research paper is entitled: 'Investigating sex differences, cognitive effort, strategy, and performance on a computerised version of the mental rotations test via eye-tracking.'"

(University of Limerick. "The sexes have equal spatial cognition skills: Men and women approach the task differently but get the same result." ScienceDaily. ScienceDaily, 27 January 2020)