The digit ratio is the length of the index finger (2nd finger) divided by the ring finger (4th finger). It correlates with the degree of androgenization or estrogenization of fetal tissues, including the fetal brain. (source)
As small bands of hunter-gatherers gave way to larger and more complex societies of farmers and townsfolk, trusting relationships had to expand beyond the circle of close kin. This larger social environment posed a two-fold problem:
For trust to evolve our ancestors must have 1) overcome the incentive to defect when involved in cooperative activity, and 2) suppressed the proclivity to use violence to take resources from conspecifics, as is seen in nonhuman primates. (Gifford, 2013)
This in turn required "social rules of governance and implicit institutions that suppressed free riding, provided rules of orderly behavior that increased cooperation by making individual behavior predictable, and also protected the property rights of individuals" (Gifford, 2013).
But what, exactly, does one do with free riders and sociopaths? Traditionally, such people were excluded from society, either by ostracism or, in more serious cases, by execution. There was thus strong selection for pro-social behavior, i.e., acting honorably and peacefully with other members of society. This selection operated even when ostracism was far from permanent or total. Shunning, public shaming, or simply a bad reputation would hurt one's chances for survival and reproduction in many ways: reduced access to community goods, discrimination on the marriage market, reluctance by others to provide assistance, and so forth.
This process of selection had genetic consequences, since nearly all behavioral traits have a heritability of 40% plus or minus 20%. There was thus removal not only of antisocial individuals from society but also of antisocial predispositions from the gene pool. The corollary was that the gene pool became dominated by pro-social predispositions, particularly empathy, compliance with social rules, and high thresholds for expression of anger.
This evolution probably occurred incrementally through small changes at many genes. This is what we see with increases in human intellectual capacity, and it is probably a general rule for the evolution of complex traits. Big changes at single genes tend to have nasty side-effects elsewhere on the genome.
A recent paper has highlighted one possible evolutionary pathway: the relative degree of androgenization or estrogenization of the developing fetus (Branas-Garza et al., 2013). By varying the ratio of one to the other, it's possible to alter a wide range of behavioral tendencies. This prenatal priming of fetal tissues can be easily measured by the "digit ratio," i.e., the length of the index finger (2nd finger) divided by the length of the ring finger (4th finger). The lower your digit ratio, the more you have been androgenized before birth. The higher your digit ratio, the more you have been estrogenized before birth.
Branas-Garza et al. (2013) found that altruistic behavior is strongest among men with intermediate digit ratios:
We analyze the association between altruism in adults and the exposure to prenatal sex hormones, using the second-to-fourth digit ratio. We find an inverted U-shaped relation for left and right hands, which is very consistent for men and less systematic for women. Subjects with both high and low digit ratios give less than individuals with intermediate digit ratios. We repeat the exercise with the same subjects seven months later and find a similar association, even though subjects' behavior differs the second time they play the game.
Different environments favor different degrees of altruism. In one setting, an altruist may be admired and enjoy preferential access to community goods. In another, the same person may be ridiculed and ruthlessly exploited. Thus, according to the context, the right balance has to be struck between altruism and selfishness:
One possible interpretation of the above findings comes from stabilizing selection. Since sharing with others is socially beneficial, selfish individuals are socially excluded and their fitness affected negatively. If individuals who are exposed too much or too little do not share with others, there is an evolutionary pressure on these non-altruistic individuals, which in turn generates an indirect evolutionary pressures on the degree of exposure to prenatal sex hormones by raising survival probabilities of individuals with intermediate levels of exposure. This hypothesis is supported by observed distributions of 2D:4D in the literature, which are universally concentrated around the median values. (Branas-Garza et al., 2013)
From one population to the next, digit ratios tend to cluster around different means, perhaps because altruism has been favored or disfavored to different degrees. This social selection may have targeted other behavioral traits, notably thrill-seeking. Kornhuber et al. (2013) have found that low digit ratios are associated with video game addition. This kind of addiction may tap into a male need for risk and adventure, which may likewise have been more adaptive in some environments than in others.
Brañas-Garza, P., J. Kovárík, L. Neyse (2013). Second-to-fourth digit ratio has a non-monotonic impact on altruism. PLoS ONE 8(4): e60419.http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0060419
Gifford Jr., A. (2013). Sociality, trust, kinship and cultural evolution, The Journal of Socio-Economics, 47, 218-227http://www.csun.edu/~hceco001/Researchpapers/Researchpapers/socialityandtrust.pdf
Kornhuber, J., E-M. Zenses, B. Lenz, C. Stoessel, P. Bouna-Pyrrou, et al. (2013). Low 2D:4D Values are associated with video game addiction. PLoS ONE 8(11): e79539.http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0079539#pone-0079539-g002