Pharyngula

The evolution of rape?

There are days when I simply cannot bear the entire field of evolutionary psychology: it’s so deeply tainted with bad research and a lack of rigor. And that makes me uncomfortable, because the fundamental premise, that our behaviors are a product of our history, is self-evidently true. It’s just that researchers in this field couple an acceptance of that premise to a deep assumption of adaptive teleology, the very thing that they should be evaluating, and produce some of the most awesomely trivial drivel.

I’ve just finished reading an article titled “Darwin’s Rape Whistle: Have women evolved to protect themselves from sexual assault?“, and it’s everything I despise about evolutionary psychology. It’s nothing but sloppy thinking and poor science propped up by a conviction that plausibility is sufficient support for certainty.

I could fulminate for a few hours over this crap, but fortunately Jerry Coyne has calmly criticized the mess, so I’ll just make a few points.

The story is that women have evolved specific adaptive responses to the threat of rape. In support of this conclusion, the author cites various studies that claim to show that ovulating women show stronger handgrip strength (the better to fight off men who want to assault their eggs with sperm), that ovulating women are more suspicious of men, that ovulating women are more likely to avoid risky behaviors, and that ovulating white women become more fearful of black men. I’m unimpressed. All of the studies involve small numbers, typically of college students at American universities (and even more narrowly, of psychology students), and all involve responses to highly subjective stimuli. When you examine the literature cited in these papers, you discover that different investigators get different results — the handgrip study even admits up front that there are conflicting results, with other papers finding no differences in performance across the menstrual cycle. None test anything to do with inheritance, none try (or even can) look at the genetic basis of the behaviors they are studying. Yet somehow evolutionary psychologists conclude that “women may have been selected during human evolution to behave in ways that reduce the likelihood of conception as a consequence of rape.”

Another way to look at it is that they are hypothesizing that women are more likely to behave in ways that invite physical attack and brutal abuse when they aren’t ovulating. That is a remarkable assertion. It also carries the strange implication that the consequences of rape can be measured by the likelihood of immediate fertilization, rather than by the toll of physical injury and emotional trauma, a peculiar thing for psychologists to neglect. Wouldn’t it make more sense to have a general hypothesis that people, men and women, who can avoid violence at any time in their life, are more likely to be reproductively successful and thereby pass on their genes to subsequent generations? That’s all they’re saying, essentially, and the straining to sex it up by tying globally useful behaviors to reproductive cycles is unconvincing.

And of course they’re looking at culturally conditioned behaviors and responses in a narrow subset of the modern human population. How likely is it that a close-knit tribe of 30 hunter-gatherers has a serious problem with rape? Wouldn’t the nature of the culture be of far greater effect in determining the frequency of pregnancy due to rape than variations in handgrip strength or variations in fearfulness in women?

Then many of the studies that are described with such enthusiastic certainty as having definitive results turn out to be subjective, pointless messes. For instance, Jesse Bering concludes that sperm competition had to have been a very significant factor in our profligately promiscuous ancestors, and that the shape of the human penis has been selected specifically for a function in extracting competitor’s sperm from the vaginal canal. Unfortunately, when you look at the actual research cited for this semen-scooping function, it’s underwhelming.

To test this hypothesis, Gallup, Burch, Zappieri, Parvez, Stockwell, and Davis (2003) simulated sexual encounters using artificial models and measured the magnitude of artificial semen displacement as a function of phallus configuration, depth of thrusting, and semen viscosity. The displacement of simulated semen was robust across different prosthetic phalluses, different artificial vaginas, different semen recipes, and different semen viscosities. The magnitude of semen displacement was directly proportional to the depth of thrusting and inversely proportional to semen viscosity. By manipulating different characteristics of artificial phalluses, the coronal ridge and frenulum were identified as key
morphological features involved in mediating the semen displacement effect.

Under conditions that raise the possibility of females engaging in extra-pair copulations (i.e., periods of separation from their partner, allegations of female infidelity), Gallup et al. (2003) also found that males appear to modify the use of their penis in ways that are consistent with the displacement hypothesis. Based on anonymous surveys of over 600 college students, many sexually active males and females reported deeper and more vigorous thrusting when in-pair sex occurred
under conditions related to an increased likelihood of female infidelity.

Got that? They have studies that show that a piston displaces fluids more effectively in proportion to the depth of movement, and that college students report that when they suspect their partner of infidelity, they screw harder. They don’t have any evidence that this behavior actually affects the fertilization rate of one partner’s sperm over another, they don’t have any indication of morphological differences in human populations that make some individuals better semen-scoopers, they don’t have any evidence that this behavior has had a differential effect in human history. It’s all a teetering pyramid of stacked “couldas” and guesses that it woulda had an influence on evolution, if there were any variation and heritable factors involved in this function.

Whenever I see this kind of tripe from evolutionary psychologists, I reflexively reach for a counter-example, and recommend that everyone read one excellent book: The Case of the Female Orgasm: Bias in the Science of Evolution, by Elisabeth Lloyd. It’s a wonderful example of solid, rigorous, scientific thinking about an evolutionary phenomenon. Lloyd analyzes a score of adaptive just-so stories about the female orgasm, carefully scrutinizing the evidence for each, and discovers that the substance is wanting. Too often investigators start with the assumption that a feature absolutely must have been selected for, or it wouldn’t be there, and then contrive elaborate rationalizations for processes that could have favored its preservation in our ancestry…and the aura of plausibility is then sufficient to conclude that it must be so, even in the absence of any supporting evidence, and sometimes even in the face of contradictory evidence.

I should reread it now — if nothing else, to wash that nasty tincture of evolutionary psychology out of my brain.