The NY Times ran an interesting article on sexology a short while ago, focusing on the differences in arousal between men and women. Like any guy, I read it hoping to discover the magic switch that turns women on, but as expected, the message is that female arousal is very, very complicated. This was not a surprise. One of the curious results, though, was that not only do men and women differ in the specificity of stimuli that induce arousal, but women’s brains (measured by self-reporting) and women’s bodies (measured by plethysmograph) don’t agree — vaginal arousal was measured when subjects saw video clips of mating bonobos and a variety of different sexual situations, while at the same time they reported a lack of interest.

It’s fascinating stuff, but I have to raise an objection. They try to use evolution to explain what’s going on.

Besides the bonobos, a body of evidence involving rape has influenced her construction of separate systems. She has confronted clinical research reporting not only genital arousal but also the occasional occurrence of orgasm during sexual assault. And she has recalled her own experience as a therapist with victims who recounted these physical responses. She is familiar, as well, with the preliminary results of a laboratory study showing surges of vaginal blood flow as subjects listen to descriptions of rape scenes. So, in an attempt to understand arousal in the context of unwanted sex, Chivers, like a handful of other sexologists, has arrived at an evolutionary hypothesis that stresses the difference between reflexive sexual readiness and desire. Genital lubrication, she writes in her upcoming paper in Archives of Sexual Behavior, is necessary “to reduce discomfort, and the possibility of injury, during vaginal penetration. . . . Ancestral women who did not show an automatic vaginal response to sexual cues may have been more likely to experience injuries during unwanted vaginal penetration that resulted in illness, infertility or even death, and thus would be less likely to have passed on this trait to their offspring.”

Evolution’s legacy, according to this theory, is that women are prone to lubricate, if only protectively, to hints of sex in their surroundings. Thinking of her own data, Chivers speculated that bonobo coupling, or perhaps simply the sight of a male ape’s erection, stimulated this reaction because apes bear a resemblance to humans — she joked about including, for comparison, a movie of mating chickens in a future study.

That all sounds very plausible, but plausibility isn’t enough — this is a perfect example of a just-so story. I’d want to see comparative data, but our closest relatives, the chimpanzees, are so different from us in sexual behavior that it would be difficult to generate an appropriate comparison. I’d want to see the causal and molecular basis for this behavior in women, but again, it’s going to be so complex that I doubt we’ll find simple relationships, let alone molecular evidence of selection. I’d want to see historical evidence that women who lacked a lubricity response to the prospect of unwanted sexual activity were more prone to injury that affected childbearing, but that doesn’t exist.

While not doubting the physiological relevance of the research, this too-willing cooption of evolutionary explanations just bugs me.

As an antidote, I have to recommend a book I’ve mentioned before, an excellent survey of evolutionary explanations for female sexuality, The Case of the Female Orgasm: Bias in the Science of Evolution^{(amzn/b&n/abe/pwll)}, by Elisabeth Lloyd. It steps through various proposed scenarios and shows the lack of legitimate evidence or, in quite a few cases, neglect of evidence that contradicts the hypotheses. It’s one of the best books around for demonstrating how rigorous evolutionary logic should be applied.

Unfortunately, that book doesn’t tell me what women want, either. The conclusion is that the female orgasm is probably an evolutionary byproduct, and that adaptive explanations are inappropriate and unjustified. I suspect the same answer applies to the work on female arousal.