In 1992, two biologists published a study purporting to prove that rape is an evolutionary adaptation in human males. Were they telling a “Just So Story?” (More)

Limits of Evolutionary Psychology, Part II: Evolved to Rape, or “Just So?”

This week Morning Feature considers the emerging field of evolutionary psychology and its potential impact on public policy. Yesterday we saw the common social science bias toward WEIRDness in a recent study on female interaction. Today we look at “Just So Stories” that presume more than the research has proved. Tomorrow we’ll conclude with why progressives must avoid “appeals to nature.”

“Psychological adaptation underlies all human behavior”

That bold claim begins a 1992 paper titled “The evolutionary psychology of men’s coercive sexuality,” published in Behavioral and Brain Sciences, wherein Randy and Nancy Thornhill claim to prove:

Thus, sexual coercion by men could either arise from a rape-specific psychological adaptation or it could be a side-effect of a more general psychological adaptation not directly related to rape. Determining the specific environmental cues that men’s brains have been designed by selection to process may help us decide which these rival explanations is correct. We examine six testable predictions against existing data: (1) Both coercive and noncoercive will be associated with high levels of sexual arousal and performance in men. (2) Achieving physical control of a sexually unwilling woman will be sexually arousing to men. (3) Young men will be more sexually coercive than older men. (4) Men of low socioeconomic status will likewise be more sexually coercive. (5) A man’s motivation to use sexual coercion will be influenced by its effects on social image. (6) Even in long-term relationships men will be motivated to use coercion when their mates show a lack of interest in resistance to sex because these are interpreted as signs of sexual infidelity. Current data support all six predictions and are hence consistent with the rape-specific hypothesis, but this does not eliminate the side-effect hypothesis, which is likewise compatible with the findings, as well as with the further evidence that forced matings increased the fitness of ancestral males during human evolution.

In other words, “ancestral males” who were able and willing to rape women had an evolutionary edge over males who were limited or limited themselves to consensual sex. Or so they argue.

It’s easy to imagine the evolutionary logic. Sexually coercive men spread their genes more widely than men who take “No” for an answer. To coerce women and survive, they would need to fight off protective fathers and brothers, and other rivals. Thus sexually coercive men would likely have been bigger and stronger on average – and thus better hunters – than men who needed consent for sex. Setting moral issues aside, rapists would have possessed more of the adaptive traits that ancestral human males needed to survive.

“Just So”

That is, of course, a “Just So Story.” That is, it proposes a seemingly logical, causal explanation without any actual evidence. We can speculate that rapists could spread their semen more widely than men who took “No” for an answer, but we can’t know how early societies reacted to such attacks, or the pregnancies and any offspring that might ensue.

Anthropologist Kim Hill spent decades studying the Aché, who still live very much like we imagine our earliest human ancestors, and he ran the numbers of the evolutionary hypothesis for rape:

He and two colleagues therefore calculated how rape would affect the evolutionary prospects of a 25-year-old Ache. (They didn’t observe any rapes, but did a what-if calculation based on measurements of, for instance, the odds that a woman is able to conceive on any given day.) The scientists were generous to the rape-as-adaptation claim, assuming that rapists target only women of reproductive age, for instance, even though in reality girls younger than 10 and women over 60 are often victims. Then they calculated rape’s fitness costs and benefits. Rape costs a man fitness points if the victim’s husband or other relatives kill him, for instance. He loses fitness points, too, if the mother refuses to raise a child of rape, and if being a known rapist (in a small hunter-gatherer tribe, rape and rapists are public knowledge) makes others less likely to help him find food. Rape increases a man’s evolutionary fitness based on the chance that a rape victim is fertile (15 percent), that she will conceive (a 7 percent chance), that she will not miscarry (90 percent) and that she will not let the baby die even though it is the child of rape (90 percent). Hill then ran the numbers on the reproductive costs and benefits of rape. It wasn’t even close: the cost exceeds the benefit by a factor of 10. “That makes the likelihood that rape is an evolved adaptation extremely low,” says Hill. “It just wouldn’t have made sense for men in the Pleistocene to use rape as a reproductive strategy, so the argument that it’s preprogrammed into us doesn’t hold up.”


Hill’s research exposes a more basic flaw in the Thornhill’s research, as anthropology professor Kate Clancy explained in Scientific American:

One of my least favorite papers is on rape as an evolved sexual strategy among humans. The abstract begins,

“Psychological adaptation underlies all human behavior.”

I still remember the photocopied version I first read of this for a class in graduate school, because it was marked up by the professor who had read it first. Next to this line, the professor had written,


Years later, the first time I taught with this paper, I pulled out that same old photocopied version to make a pdf, and I saw that professor’s comment again. And it struck me how this was one of the fundamental problems with many disciplines that tend towards the adaptationist, including evolutionary psychology. We forget that natural selection and sexual selection are only two ways in which evolution – which is really just change over time – happens. There are also things like genetic drift and mutation, which can also have a direction and also produce change. While this may drive some adaptationists into an existential crisis, sometimes there is no reason at all for a given human behavior or trait. My decision to wear navy socks today, the route I walk from one campus building to another, making cupcakes instead of cookies for my daughter’s playdate, these are behaviors we can tell adaptive stories about.

But it may not be realistic or accurate to do so. And if you do want to tell an adaptive story about it, you have to make sure the argument is pretty airtight.

Dr. Clancy continues:

There are ways to be able to be more confident about whether a trait you’re interested in has been selected (or rather, not eliminated). You can see if it conforms to these three principles:

1) The trait is variable. The number of fingers on a human hand is not significantly variable since most everybody has five. Hair length is variable.

2) The trait is heritable. Hair length is not heritable since we cut it to suit our personal and cultural preferences. Freckles are variable and heritable.

3)The trait produces variation in reproductive success. As far as I know so far, freckles do not affect how many kids you have. Voice pitch, however, is a good example of a trait that is variable, heritable, and has been shown to be correlated with the number of children a man has[.]

But even all three are not enough to prove a trait is an evolved adaptation. Dr. Clancy adds that evolutionarily useless traits may tag along with other, useful traits. A useful trait might be also be swept aside by another, even more adaptive trait. A rigorous evolutionary psychologist must show that the trait is not a tag-along, and that it offers at least as much advantage as any alternatives.

That’s a very high burden, but without meeting that burden the risk of a “Just So Story” is too high. And as Dr. Clancy emphasizes, those “Just So Stories” carry real social costs:

The bad parts of evolutionary psychology confirm what we think we already know about the world. And confirming stereotypes and calling it science tends to keep women and GLBT folk as perpetual second class citizens in this world, rather than the amazing, vibrant contributors to society they are and can be.

Evolutionary theory has been developed and tested for quite a long time, and there is a strong, reliable set of conditions we have developed to help us determine adaptive significance for a given trait. All the field of evolutionary psychology really needs is to be put to the test.


Happy Friday!