This week, I stumbled upon what may be the most amusing–if unfortunate–attacks on evolutionary psychology I’ve read to date. Quick background: evolutionary psychology is in this weird situation where almost everyone who’s studied the issue in a serious way realizes the main ideas are right, but since some people are made uncomfortable by it lots of silly remarks about it are taken seriously. I ran into this a few months when I did some source-checking on the Stanford Encyclopedia of Philosophy article on Evolutionary Psychology.

Now, Jerry Coyne has been praised by PZ Myers for a post proposing that the bad taste of semen is an evolutionary adaptation to encourage people to have vaginal sex. Coyne claims this idea “shares all the salient traits of the best ideas of evolutionary psychology,” implying since that this idea is ridiculous, therefore all of evolutionary psychology is ridiculous.

Really? Let’s compare to a classic bit of thinking out of evolutionary psychology: the reason that men decide to have sex much more readily than women is both men and women’s behavioral inclinations regarding sex are evolutionarily adaptive. This is rooted in biology: while there are many costs people can incur in order to have sex–time, money, injury from your partner, and negative social consequences–there are some costs only women have to pay. Namely: the biological cost of pregnancy, the risk of dying in childbirth, and the fact that being pregnant with one man’s child prevents you from being pregnant with another man’s. Sex, then, costs less for men for the same benefits. In particular, men can theoretically have no-cost sex, but for a woman, even if the time/money/etc. costs are zero, the biological costs remain. For men, as long as reproductive biology does not change (which it doesn’t), the rule “have sex when there are no costs” cannot fail to be adaptive.

As long as evolution can shape behavior, it would be surprising if men didn’t have sex more readily than women. But nature never had such a surprise in store for us: in culture after culture around the world, men do as expected. Among the things psychologists have done to verify this is to send attractive men and women out into college campuses to proposition opposite-sex strangers for either (1) a date or (2) immediate sex. 50% of men and women said yes to a date. 75% of men said yes to sex, as did 0% of women. Of the 25% of men who said no, many asked for a rain check or were apologetic: “I’d love to, but my fiancée is in town.” The hypothesis that women were consciously thinking about biological costs is ruled out by the fact that this was a recent study, so at least some of the women would have been on birth control pills. When psychologist David Buss lectured on this research, one female audience member reportedly stood up to say that the results could be explained on the hypothesis that men are slime. In fact, this is a tidy confirmation of the adaptation hypothesis: people were being presented with a chance for sex with most costs eliminated, and the men say yes because this is potential reproduction at no cost, while the women say “no” because the biological costs are still there.

In order for this hypothesis to be wrong, two things would have to be true: both that something prevented the expected adaptation from arising, and that some non-adaptive force caused something that looks a lot like an adaptation. This is a possibility, but the adaptive hypothesis is far more likely.

The problem with Coyne’s hypothesis about semen is this: If a woman discovers a man has bad-tasting semen, is she more likely to have intercourse with him, or more likely to refuse any kind of sex? The answer isn’t obvious in the way that the facts about reproductive biology are obvious. Also, while the facts of reproductive biology weren’t any different when humans were hunter-gatherers, this sort of thing isn’t as fundamental to the way humans are built. Finally, while there’s no easy non-adaptive hypothesis for men’s sexual inclinations, there is an easy non-adaptive hypothesis for the taste of semen: the bad taste is just a byproduct of making an efficient tool for egg-fertilization.

As one of Coyne’s commenters points out, the non-adaptive hypothesis can be checked by seeing if the composition of semen in primates that don’t perform oral sex is basically the same. Coyne misses the point when he suggests that female taste buds could be what evolved, because while men have an incentive to always take low-cost measures to get women to have sex with them, women can’t have a rule that always pushes them towards sex (remember that from an evolutionary point of view, oral sex is more like making out that real sex, something that is unchanged by Dan Savage’s insistence that oral sex is real sex.)

Now, this is all very amusing, and it gave me a chance to write an evolutionary psychology 101 post, but it’s also a little distressing to see this coming out of a serious intellectual. I had been looking forward to reading Coyne’s book Why Evolution is True, which by all other indicators seems to be a good book. But I have hope: Coyne has a good post criticizing the NCSE. Go read that now.