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

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

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

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

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

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

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

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

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

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

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

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

Metacarpals and phalanges respond differently to prenatal androgens

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

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

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

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

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