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The digit ratio is the length of the index finger (2ndfinger) 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

(Republished from Evo and Proud by permission of author or representative)
 
• Category: Science • Tags: Altruism, Antisocial Behavior, Empathy 
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  1. Anonymous • Disclaimer says:

    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.

    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:

    It's not clear that altruism, as opposed to behavior that appears like altruism, actually evolves in these more complex cosmopolitan environments.

    The evolution of communication, as opposed to manipulation, is very much akin to the evolution of altruism. It is subject to the same exploits in a cosmopolitan environment. So manipulation via signaling taking on the appearance of communication is far more common in complex, cosmopolitan environments than in the environment in which communication evolved. There's an evolutionary arms race to defect faster than the other guy while maintaining keeping active his obsolete instincts to think communication is still viable.

    The end result of civilization, of complex cosmopolitan societies of farmers and townsfolk beyond the circle of close kin, is a bifurcation: 1) communication terminates as a genetic capacity and/or 2) communication takes a form similar to that in eusocial species with manipulative pheromone signalling even as civilization takes that form.

  2. Luke Lea says: • Website

    My ratio differs markedly between my right and left hands. What does that mean?

  3. It would seem that as groups get larger, they need to get more draconian to enforce non-disruptive behavior, or altruism and co-operativeness needs increase …

  4. Anonymous • Disclaimer says:

    I larger societies people can't keep track of everyone's history or reputation. And people know this so there is greater opportunity to defect.

  5. Sean says:

    Black Africans have the lowest digit ratios. Given a female farming system men can be polygynous. Most male violence boils down to fighting over women (or just stealing them).

    In the steppe-tundra scenario there would be direct selection against men trying to get extra women; they just couldn't provision them.

    As I see it, steppe tundra Europeans would be pre adapted to pro-social predispositions through being less selected for male-male competition for women.

  6. Anonymous I says: • Website

    Looking around, it doesn't seem that the 2D:4D ratio is a marker for prosociality, but rather for emotionality (sentimentality, fearfulnesds, dependency, and anxiety).

    For instance, Shaw's 2013 article, "The effect of prenatal androgen exposure on the development of neural reactivity systems: A study of the HEXACO Personality Inventory" yields a rho between .12 and .20 for Emotionality, but between no significant relationships for Honesty-Humility.

    The only suggestion I could find of a relationship to genuine antisociality (rather than traits related to emotionality) was one in the *opposite* direction – feminized hands were related with psychopathy in Blanchard & Lyon's (2010) study, "An investigation into the relationship between digit length ratio (2D: 4D) and psychopathy."

    If there is a U-curve for altruism, this would suggest that altruism depends on normal functioning, not androgens or oestrogens. Individuals with disrupted development or high genetic load would display this in various abnormalities, such as odd digit ratios, fluctuating asymmetry, or the so-called "minor physical anomalies," which are strongly associated across numerous studies with criminality. Indeed, it wouldn't surprise me if an extreme 2D:4D ratio were to qualify as one such minor physical anomaly.

  7. Anonymous I says: • Website

    By the way, Peter, when you claim that most behavioral traits are 40% +/- 20% heritable, are you speaking off the cuff, or do you have some meta-analysis in mind? The psychological traits I've seen have heritabilities closer to 55% +/- 20%.

  8. Gottlieb says:

    My index finger is longer on both hands. I try to be social, but always end up making me antisocial because I do not understand people and people also do not understand me.
    I recently read that stutterers have the largest corpus callosum and have greater activation of the right side of the brain. I'm stuttering and probably also have two common neurological traits among this population.
    PSYCHOPATHS also has the largest corpus callosum but it is thinner than average.

  9. Gottlieb says:

    Oops, I emphasized the word psychopath, sorry. Damn translator,'ll still kill you. Scout's promisse.

  10. Anonymous • Disclaimer says:

    "My ratio differs markedly between my right and left hands. What does that mean?"

    witch!!!!!!!!!!!!!!!!!!!!!!!!

    (not really)

  11. Anonymous • Disclaimer says:

    http://www.viewzone.com/finger2.jpg
    High 2D:4D (both hands)

    I'm an ex-bleeding heart: fighting world hunger, world poverty etc.

    My altruism is now only for R1B1 and R1A1.
    I can share with other people my middle finger by showing it, or my saliva from the distance of 1 m.

  12. Anon,

    Altruism already existed when humans were organized in small bands, but it was limited to close kin. Such altruism makes genetic sense, being a form of fitness maximization. With more remote kin, altruism has to be maintained by other selection pressures.

    Luke,
    It depends on which hand you have the higher ratio: "there were negative associations between 2D:4D and testicular function, and men with lower 2D:4D in their right compared to left hand had higher testosterone levels than men with higher 2D:4D in their right compared to left hand."

    http://www.asiaandro.com/archive/1008-682x/6/211.htm

    Beyond Anon,

    Either draconian or totalitarian. Larger societies require more compliance with social rules if they are to exist as peaceful, functioning entities. As a result, the mental space occupied by social rules, and the willingness to comply with them, creates a potential for ideological abuse. If you seize control of this mental space, you can make people– at least, most normal people–do almost anything, as long as it is perceived as normative behavior.

    Sean,

    Well, OK, but evolution didn't stop with the end of the Upper Paleolithic. What happened in the Upper Paleolithic, particularly among the complex hunter/fisher/gatherers around the North Sea and the Baltic, helped to create behavioral predispositions that would be more useful later on, in even larger and more complex social environments.

    Anon,

    Pro-sociality isn't just about honesty. It's actually a fairly large package of predispositions. Empathy is probably just as important.

    Gottlieb,

    I think perceptions are critical. Sociable people do not necessarily understand others better, but they project an image of understanding.

  13. Anonymous I says: • Website

    "Pro-sociality isn't just about honesty. It's actually a fairly large package of predispositions. Empathy is probably just as important."

    Peter, that large package of prosocial predispositions you describe *is* Honesty-Humility. This is a broad trait relating not only to honesty or humility in specific, but to active prosociality as well as empathy. All of these traits are intercorrelated.

    For example, consider a study by Laboff et al, (2012) "Humble persons are more helpful than less humble persons: Evidence from three studies," from The Journal of Positive Psychology. They found that measures of humility alone predicted state empathy (r = 26) and trait empathic concern (r = .46), as well as actual helping behavior in two experimental conditions.

    Prosocial behaviors and attitudes like truth telling, greed avoidance, fairness, empathy, kindness, and humility all cluster together under the broad trait psychologists refer to as Honesty-Humility, just as verbal ability, pattern recognition, and working memory, cluster together on conventional intelligence tests.

    Of course, some aspects of prosocial behavior are related to other broad traits; empathy is also related to "Emotionality." But on the other hand, it is quite possible to have empathy while still exploiting others; I've read that rapists feel more empathy for women that nonsexual offenders. Empathy alone isn't the issue; Honesty-Humility is. When you talk about the broad cluster of traits relating prosociality, you are talking about Honesty-Humility.

  14. Anonymous • Disclaimer says:

    Altruism already existed when humans were organized in small bands, but it was limited to close kin. Such altruism makes genetic sense, being a form of fitness maximization. With more remote kin, altruism has to be maintained by other selection pressures.

    Yes, altruism evolved in small bands of close kin. In cosmopolitan environments, it is subject to exploits. Manipulation via signaling taking on the appearance of altruism is far more common in complex, cosmopolitan environments than in the environment in which altruism evolved. There's an evolutionary arms race to defect faster than the other guy while maintaining keeping active his obsolete instincts to think he is still in an older environment of small bands of close kin and that altruism is still viable.

    The end result of civilization, of complex cosmopolitan beyond the circle of close kin, is a bifurcation: 1) altruism terminates as a genetic capacity and/or 2) altruism takes a form similar to that in eusocial species with manipulative pheromone signalling even as civilization takes that form.

  15. Anonymous • Disclaimer says:

    Peter,

    If you haven't seen it, a roundup of the Nicholas Wade controversy:

    http://occamsrazormag.wordpress.com/2014/05/06/roundup-of-book-reviews-of-nicholas-wades-a-troublesome-inheritance/

    Also, some guy is attacking you in the comments here:

    http://inthesetimes.com/article/16674/the_genes_made_us_do_it#comment-1383439901

    ..

  16. Anonymous I says: • Website

    "Also, some guy is attacking you in the comments here:"

    Oy lads – let's get 'im!

  17. Ben10 says:

    What about the toes?
    Mines II and III are halfway fused.
    I saw a china guy in sandals, his toes were as long as mine fingers! WTF?

  18. Gottlieb says:

    ''Gottlieb,

    I think perceptions are critical. Sociable people do not necessarily understand others better, but they project an image of understanding. ''

    Strongly agree, thanks.

    Take advantage of your generosity I wish you could give me any chance of success in relation to the theory that I believe, is being pioneered in work, trying to explain the exceptional and specialized Ashkenazi intelligence by means of laterality patterns. Seems to be left-handed or ambidextrous is strongly related to unusual brain morphology especially with brain symmetry instead of the usual asymmetry specializes in the left side of the brain. This would explain the high incidence of mental disorders in these populations compared to right-handed people. It is known that the extremes of skills can be found in higher proportions within these populations, precisely because of the symmetrical brain morphology. It would be interesting to study the differences in brain lateralization between human races and my guess is that the percentage of left-handers among Ashkenazim is a reflection of a greater amount of brains connected in a different way than usual, the human pattern.
    Another theory came me to mind now is whether the Aborigines could not be a kind of savant with very low iq performance but with excellent spatial skills.

  19. How does empathy have construct validity independantly of the rest of the 'social brain'? It doesn't and it can't.

    Abandon the concept.

  20. Anonymous • Disclaimer says:

    I actually think Honesty-Humility related traits are probably less important for high level prosociality (of the sort that leads to a peaceful society) than simple non-retaliation, a secure personality yet respect for the law, but…

    For example, consider a study by Laboff et al, (2012) "Humble persons are more helpful than less humble persons: Evidence from three studies," from The Journal of Positive Psychology. They found that measures of humility alone predicted state empathy (r = 26) and trait empathic concern (r = .46), as well as actual helping behavior in two experimental conditions.

    So how much of the state of empathy and trait of empathic concern are NOT explained? How much of heritable trait empathic concern is left over after trait Honesty-Humility is accounted for?

    If there's quite a lot, then why obsess about Honesty-Humility rather than just look at the trait of interest?

    Since you discuss offenders – http://www.qub.ac.uk/schools/media/Media,367250,en.pdf

    Differences on Emotionality, Openness, Conscientiousness and Extraversion are all about as big a "H" between offenders and non-offenders, while HEXACO Agreeableness falls short (differences might come into play with a larger sample size – offenders seem quite likely to me to be angrier, more impatient and less forgiving).

    Being prosocial is not all about being honest or humble, but generally a factor of an socially effective, rational, self assured personality. So there is no reason that evolution would have selected particularly much for Honesty-Humility when promoting prosociality.

  21. Anonymous • Disclaimer says:

    I'm not sure why honesty would necessarily be pro-social. Social living often requires people to be less honest in order to maintain smooth social relations and reduce social conflict. People who are very honest and forthright tend to be less sociable. The most extreme example would be autistic people who are extremely honest and direct yet completely unsociable.

  22. Anonymous I says: • Website

    It is a replicated empirical finding that these traits under discussion dovetail. One could similarly postulate that verbal and mathematical ability would interfere with one another, so that individuals tended to be good at one or the other, but not both. But it turns out that this simply is not the case.

    That stated, I think what the bluntness you're describing is probably more related to a different factor, Agreeableness vs. Anger. I doubt that people who are high in H factor are always forthright; they probably tend to hold silent when bluntness might be impolite. Moreover, they probably also dare to think fewer uncharitable thoughts to begin with. They score lower in Machiavellianism than people low in H do.

  23. Anonymous • Disclaimer says:

    Honesty goes together with "pro-social" humility, on average, but as Anonymous 14:54 correctly states, the Honesty-Humility construct explains little of the pro-social phenotype.

    I.e. visual and verbal intelligence may not "interfere" with one another, but each might be little able to predict overall problem solving ability (not the case with these, because g, but H is very, very far from being a g for pro-social ability).

  24. Anonymous I,

    I agree that these different pro-social predispositions tend to cluster together. Are you suggesting some kind of genetic linkage? I think the linkage is more at the level of natural selection. The same selection pressure that favors one form of pro-social behavior probably favors others.

    The figure of 40% plus or minus 20% comes from Robert Bailey of Northwestern. See: Conceptual Challenges in Evolutionary Psychology: Innovative Research Strategies, p. 24

    Anon,

    Actually, that e-book was peer-reviewed.

    Ben,

    The digit ratio effect seems to be limited to the hands and especially to the right hand. Don't know why.

    Gottlieb,

    I don't feel qualified to comment. Cochran and Harpending's theory is that strong selection favored alleles that are advantageous for mental functioning as heterozygotes but not as homozygotes. Most of the diseases specific to Ashkenazim seem to be concentrated in a single metabolic pathway (lipid storage) that is related to cognitive performance.

  25. Gottlieb says:

    ''Gottlieb,

    I don't feel qualified to comment. Cochran and Harpending's theory is that strong selection favored alleles that are advantageous for mental functioning as heterozygotes but not as homozygotes. Most of the diseases specific to Ashkenazim seem to be concentrated in a single metabolic pathway (lipid storage) that is related to cognitive performance.''

    Peter Frost,
    well, my approach is not based to analyse very specific genetic subject but to go to whole and more easily understandable view. I have by personal will collected anedoctal evidences that prove more incidence of left handedness in ashkenazim than other populations and inclusive, than other jewish populations. Some common diseases as crohn syndrome, breast cancer and auto immune diseases. Also i have found some anedoctal evidences about more frequence of genes in ashkenazim that predisposed to schizophrenia as well to bipolar disorder. left handedness or some subgroup of left handed people to seems to be cognitive advantages as more creativity and intelligence. I believe that ashkenazim selected more some of this subgroups.
    Even the profile cognitive of ashkenazim is assymetrical. I have the feeling that this kind of cognitive profile is quite common in lefties and in ambidexterous.
    But, thanks by the atttention professor!

  26. Gottlieb says:

    ''Gottlieb,

    I don't feel qualified to comment. Cochran and Harpending's theory is that strong selection favored alleles that are advantageous for mental functioning as heterozygotes but not as homozygotes. Most of the diseases specific to Ashkenazim seem to be concentrated in a single metabolic pathway (lipid storage) that is related to cognitive performance.''

    Peter Frost,
    well, my approach is not based to analyse very specific genetic subject but to go to whole and more easily understandable view. I have by personal will collected anedoctal evidences that prove more incidence of left handedness in ashkenazim than other populations and inclusive, than other jewish populations. Some common diseases as crohn syndrome, breast cancer and auto immune diseases. Also i have found some anedoctal evidences about more frequence of genes in ashkenazim that predisposed to schizophrenia as well to bipolar disorder. left handedness or some subgroup of left handed people to seems to be cognitive advantages as more creativity and intelligence. I believe that ashkenazim selected more some of this subgroups.
    Even the profile cognitive of ashkenazim is assymetrical. I have the feeling that this kind of cognitive profile is quite common in lefties and in ambidexterous.
    But, thanks by the atttention professor!

  27. Anonymous I says: • Website

    …It seems my last response is missing. Given its content, I can't imagine Peter would have deleted it, but it took quite some time to write. This is probably the second time it's happened; maybe I should start saving my posts beforehand!

    Let me just respond to:

    "I agree that these different pro-social predispositions tend to cluster together. Are you suggesting some kind of genetic linkage?"

    No no, just phenotypic.

    "as Anonymous 14:54 correctly states, the Honesty-Humility construct explains little of the pro-social phenotype."

    This is wrong. I agree that his study seems to show that, but across studies, personality constructs other than H give contradictory findings depending on the sample and the way prosociality is operationalized. For just one example, see table 2 of "The prediction of Honesty–Humility-related criteria by the HEXACO and Five-Factor Models of personality" at http://www.sciencedirect.com/science/article/pii/S0092656608000469 . Only H consistently predicts pro- and antisocial behaviors.

  28. Anonymous • Disclaimer says:

    Yet, H is not particularly distinguished in a sample of prisoners (a particularly anti-social subsample) compared to the other factors.

    Also, Ashton and Lee themselves (the two researchers who actually developed the HEXACO) are quite comfortable to say

    "Noted in this section, the H and A factors are interpreted as two aspects of a tendency toward reciprocally altruistic behavior. We have also proposed that the E factor (again, Emotionality) represents a tendency toward kin altruism and other behaviors that reduce harm or gain help for one’s kin (see detailed discussion in Ashton & Lee, 2007).

    Therefore, these three dimensions contribute independently to an overall tendency to engage in prosocial or altruistic versus antisocial or antagonistic behavior. Consistent with this interpretation, the HEXACO-PI-R facet scale of Altruism shows a strong projection on the vector defined by the combination of high H, high A, and high E, as do adjectives such as kind, sympathetic, and generous"

    A person who is suspicious, angry and unforgiving (low A), tough and unemotional (low E), socially self isolating (low X), lazy and non-self disciplined (low C), person would not manage to be effectively "pro-social" simply through a high H factor.

    Ashton also has been clear that facet level data yield predictive variance beyond the broad factors –

    http://www.subjectpool.com/ed_teach/y4person/2_facets/refs/Ashton_2001_why_use_facets.pdf. And the predictive variance of H itself, even if it may in some studies be more impressive and statistically significant than any other individual factor or facet, is far from total.

  29. Anonymous • Disclaimer says: • Website

    This post is great.your article is important and helpful for social.This post give more new informative information for social.Thanks a lot for share your great post.

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