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.
References
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, 218-227
http://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
