The (Lack Of) Genetics of Rape

There has been a flurrylet of blog posts recently to do with, in various forms, genetic determinism and adaptationism; two ideas that together form a general philosophy that the traits that make up human biology are largely determined by human genetics, and that these traits (and the genes that underlie them) have come about as the direct result of natural selection. In particular, people have been talking about evolutionary psychology, which involves explaining human behaviour in terms of adaptations. I felt like commenting, which I guess you have already infered by the existence of this post.

Jerry Coyne pointed out two articles in the popular press about genetic determinism, the first by the journalist David Brooks, and the second by the journalist Sharon Begley. Jerry Coyne himself wrote a detailed takedown of evolutionary psychology as early as 2000. There is a lot to say about Sharon Begley’s piece in particular.

Sharon Begley on Rape

Sharon Begley’s article is mostly taking apart the book A Natural History of Rape by the unfortunately named Randy Thornhill; a book that attempts to explain rape, as well as a other sexual behaviours, as adaptations - rape gives men who cannot get access to females the ability to spread their seed. The book was pretty comprehensively demolished at the time (e.g. Kenan Malik’s review), and has a majority of very negative reviews on the Amazon US and UK (though a distressing number of those are people who believe that explaining a trait is the same as justifying it).

Sharon Begley in particular points out a study by the anthropologist Ken Hill, in which he performed did a cost-benefit analysis of rape in the Aché people of Paraguay. I personally don’t see why we should especially expect Aché society to be particularly representative of an ancestral human population, especially if we are thinking about a population from hundreds of thousands of years ago, but it certainly does demonstrate that there is no particular reason to think that rape would be adaptive in humans. The article goes on to discuss plenty of other data that goes against what is predicted by Thornhill’s theory, in particular emphasising how, while their predictions come true in some populations, they do not in others, and generally suggests that human culture is far too dynamic to make hard-and-fast adaptations useful (she also talks favourably about behavioural ecology, but she seems to have misunderstood exactly what it is; behavioural ecology is pretty closely related to evolutionary psychology).

I should point out that I do not believe that the worst excesses of evolutionary psychology apply throughout the entire field. I remember being very impressed by the remarkable book Human Sperm Competition, which was a book based on a large store of biological data, from humans and other mammals. It even included asking women to recapture sperm that was washed back from the vagina after sex, and cross-correlating what they found in it with questionnaire results. In general, the book highlighted a number of general themes of human sexuality, which it tied together in a convincing manner with studies into other primates and more distance mammals. However, it is precisely this lack of extensive data that characterises the type of evolutionary psychology that tends to end up in the popular media, as collecting data takes time and effort, whereas writing popular books based on speculation is fast and easy, generally with a higher payoff.

The New Methodology

Now, all these objections are perfectly good, as far as they go. But the thing that really irks me, as a geneticist, about evolutionary psychology is that they have generally completely failed to use the extensive tools of modern genetics that have actually allowed us to test .

The hypothesis that rape is an adaptation is not inherently ridiculous. Some people might actually consider it likely; though the complexity of the psychology rape and sexual violence, and the huge influence of society expectation and openness toward rape, along with the not-insignificant numbers of male date-rape victims, makes it seem unlikely to me that a ‘spread your seed’ hypothesis can account for rape in any comprehensive way.

For instance, suppose we found a unique rush of hormones that often accompanied coercive sex and sexual violence. Suppose we also found a particular regulatory pathway that triggered this unique combination of biochemical markers, that this pathway was co-expressed with sexual violence throughout different human populations, and, under very carefully controlled conditions, triggering this regulatory pathway increased sexual violence or willingness to rape a woman. This would show that there was a direct genetic underpinning to rape. Suppose we then looked for evidence of selection on this pathway, either in recent human history or in deeper evolutionary time, and found that selection in the direction of promoting this pathway had been acting. We would then have a firm, well researched piece of science strongly indicating that rape was a adaptively selected genetic trait.

Now, we don’t have that. Of course we don’t have that, what we have is vague pontificating without any supporting evidence. No evolutionary psychologist has sat down and attempted to find neurological or biochemical associations of sexual violence (though neuroscientists have, even if some of it looks a bit dodgy to my untrained eye), or to attempt to look at genetic variation in the sort of traits that are associated with rape.

But it is not as if this sort of research is impossible to do: geneticists do it all the time. There are definite, established experiments you can do to test whether the is a strong genetic influence on a trait: we can control for strong societal effects like class by just looking at the difference in trait co-occurrence between identical and non-identical twins. If you can predict obesity far better by knowing the obesity status of a identical twin than you can by knowing the status of a non-identical twin, you know that genetics is in play. This has been done in depth for obesity (e.g. here), and we know that there is a definite genetic effect there (and it appears to be far stronger than you might expect); we have found a couple of genes, notable FTO and MC4R, that contribute to obesity after population structure (class, location etc) are controlled for. In general, our ability to predict obesity based on genetics along is getting close to our ability to predict obesity based on class: as a quick back-of-the-envelope calculation, the bottom 25% socioeconomically have around a 3-fold increase in obesity risk, whereas the increase for the highest 25% in terms of genetic risk have a 2-fold increase (based on data from these three papers); expect both of these numbers to go up as we understand both the cultural and the biological underpinning better.

MC4R is a particularly good example, it is the strongest genetic effect we have, accounting for around 6% of variation in obesity, it is important in the biology of feeling full, and has been shown to have been under increasing purifying selection during human evolution (i.e. mutations that stop you feeling full are weeded out by natural selection, see this paper). This is an example of finding a gene that contributes to behaviour (in this case, overeating), showing that it has been under evolutionary selection (in this case, to stop you overeating), and has a known, measured mechanism. We also have an estimate of how much it influences behaviour: the figure of 6% for a given genetic contribution is pretty damn large by the standards of genetic variation, compared to probably about 10% for known effects of class (based on the same paper I cited above, which is from a German town of Aachen - both figures will of course vary from population to population). The vast majority of variation is still unexplained, for virtually every human trait. Anyone who says that they can explain human variation, be it in terms of genetics of in terms of culture or society, is deluded.

The Difference

Anyway, the point I am trying to make is how divorced the methods of bad evolutionary psychology, which basically amount of speculation and just-so stories with a few case-studies thrown in, differ from the methods of those that study the genetics and evolution of complex traits. It is very informative to compare the tentative conclusions of profession geneticists, many of whom have made explicit calculations of the genetic contribution to human biology, and the overarching and overconfident conclusions of the popular evolutionary psychologists, who are mostly unconstrained by hard biological data.