In his recent blog post, evolutionary biologist and sleeping-with-mean-people advocate Jeremy Yoder tries his best to win an argument by shoplifting the conclusion rather than paying for it with good points. If you’re just tuning in, our debate was initiated when he bombed my Don’t Sleep With Mean People campaign Pearl Harbour style, prompting me to respond with a challenge: since Yoder claims he is keen to support a project of mine that teaches people about evolution, he should concede that this project is a worthy one and pay up. One thing I didn’t expect to have to do is teach an evolutionary biologist about evolution, but since this one keeps digging himself deeper, here goes.
In his original post, Yoder argued that the DSWMP slogan would never work because it assumes “the major component of variation in meanness is explained by genetics” and because the meanest people he could imagine – Rush Limbaugh, Simon Cowell, and Anne Coulter – would always be rationalized as charming and kind by their lovers. Although I deliberately left the definition of “mean” open-ended (ie “don’t sleep with homophobes” is a fair interpretation), I had less innocuous forms of meanness than social conservatism in mind, things like violence, aggression, cruelty, coercion, etc, the kinds of behaviours that incur felony charges and are overwhelmingly a male-perpetrated phenomenon. In his new post, Yoder’s argument is not that male violence isn’t an adaptation; rather, he argues that our violent tendencies have been so completely drilled into us by natural selection that they show insufficient genetic variation for selection to act on:
It may well be that there is something programmed into the development of human males that makes us, as a group, more likely to be violent once we’ve reached maturity—and, certainly, the development of boys into men is controlled by our genes. But natural selection (or artificial selection promoted by a nerd-rapper) needs genetic variation in order to operate. The fact that every male becomes more violent after puberty doesn’t tell us anything at all about whether individual differences in the magnitude of that change—differences that can be dramatic, both across and within cultural groups—are due to individual genetic differences. And if you don’t know that, you really can’t claim that you could select for reduced masculine meanness.
He’s right that a complete lack of individual genetic differences in proneness-to-violence would be a death-blow for my campaign, but luckily for me and all the other peaceniks who support the DSWMP credo, Yoder simply didn’t bother to look up any of the evidence.
A brief search of “genetics” “heritability” “aggression” and “violence” turns up compelling studies from Applied Genetics and Neuroscience, but the best evidence comes from Behavioural Genetics. Behavioural geneticists have devised excellent methods for estimating the amount of heritable variation in personality and behaviour, and proneness to violence is definitely part of their repertoire. They measure heritability by looking at the differences between pairs of fraternal and identical twins raised together and raised apart, and by looking at the similarities and differences between adopted siblings raised in the same home, thus controlling for environmental vs genetic influences. The New York Times summarizes one research finding (Kevin Beaver’s) like so:
He has tried to measure which circumstances — having delinquent friends, living in a disadvantaged neighborhood — influence whether a predisposition to violence surfaces. After studying twins and siblings, he came up with an astonishing result: In boys not exposed to the risk factors, genetics played no role in any of their violent behavior. The positive environment had prevented the genetic switches… that affect aggression from being turned on. In boys with eight or more risk factors, however, genes explained 80 percent of their violence.
When Yoder falsely criticized me for assuming that “the major component of variation in meanness is explained by genetics” I responded that he was overstating the case: all we need is for the variation to have some genetic basis, even a minor component, and DSWMP would have an effect. Ironically, the evidence suggests that in high risk populations the difference between a violent thug and a good citizen shows 80% heritability! I’m also willing to bet that among the “risk factors” being measured, they didn’t include the presence or absence of conscientious young women promoting the sexual attractiveness of peaceful conflict resolution. “Don’t Sleep With Mean People” is another way of saying: if women take a stand and proclaim that they won’t be turned on by aggression, that could prevent the genetic switches behind aggression from being turned on.
Yoder’s recent argument also contains an interesting analogy that I’d like to expand on. He likens “being a violent male” to “having five fingers on each hand,” with the implication being that it might in fact be entirely environmental conditions that cause any variation from the norm:
Consider another trait that is invariant across human populations: almost everyone on the planet has five fingers on each hand. Five-fingered hands absolutely have a genetic basis… But knowing this doesn’t tell us, when we see a man with only four fingers, that there’s a genetic basis to the variation he represents. Environmental conditions can have profound effects on human skeletal morphology.
As a thought experiment, imagine a hypothetical population of humans where the culturally-favoured method of male-on-male competition is knife fights. Imagine also that knife fights in that population are pretty much the only way a man ever loses fingers, and only the winners are missing fingers, because the losers are all dead (good fighters enjoy a straightforward fitness advantage). Finally, imagine that not all men get into knife fights with equal frequency; some prefer to compete instead with poems or music or by arguing publicly over scientific nuances, and some prefer a combination of strategies. If the tendency to get into knife fights were linked to some stable, heritable personality trait, for instance (dis)agreeableness – and if the heritable variation in that trait were estimated at, say, around 41% or 42% – then having four fingers or less would show significant heritable variation!
Of course, all babies without defects would still be born with five fingers, but the odds of losing one or more fingers after puberty would be strongly influenced by genes. Twin studies would reveal that identical twins were more likely to end up with correlated finger numbers than fraternal twins, and adopted siblings would more closely resemble their biological parents than their adopted parents in number of fingers lost over a lifetime. This is just to point out that Yoder may have his thumb unfairly pressed on the scale in favour of “environmental” factors, which might in some cases be a part of our extended phenotype.
Now imagine some enterprising troubadour, concerned both for the welfare of society (such a pointless waste of fingers!) and for his own welfare as a better wordsmith than a knife fighter, were to write a semi-serious but whimsical song encouraging women not to sleep with men who are missing fingers, which could serve as a proxy for the genes behind the fighting. The women might still be attracted to knife fighters, but also to troubadours and scientists and other types as well, and they might be sick of all the bloodshed. If the song quickly caught on and the women (and gay men) could be heard to sing “He’s no good for kissing, his fingers are missing” on every street corner, knife fighting might disappear from the population, even if the genetic variation remained. Changing the cultural triggers would have the same phenotypic effect as changing the genes. Those low-agreeableness high-aggression low-compassion genes might persist in the gene pool for some time, grudgingly muted by the new culturally-imposed standard of good behaviour, but without their fitness advantage they would eventually be lost to drift.
In the same way, my DSWMP campaign targets the intersection of culture and genes. It also assumes that the effects of genes on human behaviour, and the effects of sexual selection (intentional or otherwise) on gene frequency are probabilistic rather than one-to-one deterministic. If the campaign decreases the likelihood of genes “for” violence (ie genes that increase the probability of violence in certain contexts) being either expressed or passed on at all, I will be satisfied, and if it teaches a few people about evolution in the process, even better.