I’ve talked a few times about Richard Prum’s new book The Evolution of Beauty, which asserts that female choice drives the evolution of male sexually dimorphic traits in nature, like fancy plumage and mating dances. That’s not in doubt, but Prum’s big claim is that what females are seeking in mates are arbitrary “aesthetic” features of males not connected with their vigor or genetic endowment. He calls this the “Beauty Happens” model, and it’s a version of the “runaway model” of sexual selection proposed by Ronald Fisher, Russ Lande, and Mark Kirkpatrick.

While Prum’s book has its good bits, it’s pretty misleading about the evolution of sexual behavior, as I pointed out on a review on this site.  As I wrote,

The book’s problem is that it is tendentious. Prum doesn’t describe the issues with his favored runaway model; he mistakenly regards it as a “null model” against which other models must be tested since, he wrongly claims, it makes no assumptions (he also claims that his null model can neither be proven nor disproven, which makes it non-scientific); he neglects other forms of sexual selection; he does not recognize that various models can work together and likely do work together; he ties “good genes” models to eugenics and even Nazi eugenics, unfairly tarring sexual selection theory with the residue of an unsavory past; and he claims that female choice of mates, which he calls “sexual autonomy”, somehow vindicates feminism in our own species.

Not only does Prum take a very one-sided view of how female mate choice evolves, but lards his book thickly with the idea that female choice in animals (mostly birds) is somehow a vindication of feminism in humans. This is, of course, known as the Naturalistic Fallacy. Not all female animals have sexual autonomy, and one could draw very different lessons from observing deer or other species in which males compete with each other, and the female gets the winner. Alternatively, there are animals like bedbugs, in which female “sexual autonomy” is obliterated by “traumatic insemination”, in which males inject sperm directly into the female body cavity, sometimes injuring or even killing her. Equal rights and opportunity for women should come from moral contemplation, not from observing what animals do. If you tie your biology so closely to your ideology or morality, you risk having to alter your ideology when we learn new facts that aren’t relevant. Should a student of bedbugs be opposed to women’s rights?

Prum also ties “good genes” models (those models in which females choose males because they have ‘good genes’ that will improve the fitness of their offspring) with Nazi eugenics. As Prum says on pp. 328-329:

“To permanently disconnect evolutionary biology from our eugenic roots, we need to embrace Darwin’s aesthetic view of life and fully incorporate the possibility of nonadaptive, arbitrary aesthetic evolution by sexual selection. . . Accordingly, evolutionary biology should adopt the nonadaptive, Beauty Happens null model of the evolution of mating preferences and display traits by sexual selection”.

That’s just ridiculous. We reject scientific theories not because they have antecedents in politically unsavory views and behavior, but because they’re not supported by the data. Connecting the good genes models with Nazi eugenics is a sleazy and musteline tactic. But it’s this connection between Prum’s theory and politics on the one hand and feminism on the other that has made his book quite popular—in my view, way too popular given its problems. I’ll have more to say about this at a future date.

Throughout the book, Prum suffers from what I call The Big Idea Syndrome (TBIS): the view that his idea has nearly universal explanatory power. (A similar victim of TBIS was Lynn Margulis, who thought that endosymbiosis explained nearly everything about biology, including the formation of new species).

One of the things that Prum proposes, toward the end of The Evolution of Beauty, is that homosexuality in humans also evolved as a result of females seeking to exercise sexual autonomy—their evolutionary “need” to have free mate choice. Prum explains this in a short  Big Think talk (transcript here):

Here’s a bit of the transcript, with my comments:

So individuals that are attracted to the same sex are frequently imagined to evolve because they provide help to their kin, that is, if there are some people in any social group that are non-reproductive because of their sexual preferences then they will be helping with raising of their nieces and nephews. This is sort of the “helpful uncle” hypothesis. The problem with that idea is that it should actually lead to a kind of asexual phenotype or an asexual behavior; it doesn’t actually describe the evolution of sexual attraction itself.

What he means at the end is “same-sex sexual attraction”. And yes, it’s possible to become nonreproductive if by so doing you actually gain fitness by taking care of your relatives (this is possible if your own sacrifice of your genes by not reproducing is more than compensated by the passing of your genes through the relatives you tend). This in fact is one explanation for the sterility of worker bees. But  Prum’s right that becoming nonreproductive doesn’t necessarily explain why the “gay uncle” is gay. He could just be asexual. And I don’t accept this hypothesis, but I’m not sure there is an evolutionary explanation for homosexuality. Nevertheless, Prum ignores evidence that is in favor of the “gay uncle hypothesis“, including some surveys in which gay men seem more willing than straight men to help relatives. (That’s only weak evidence, of course, but it’s evidence.)

Here’s his theory about how homosexuality (including lesbianism) evolves by the “Beauty happens” theory (my emphasis):

Well the aesthetic view of evolution proposes that we should put subjective experience—that is, the nature of animal and human desire—at the center of our scientific explanation. So in order to explain same sex attraction in people we need to actually ask: how could same sex attraction actually evolve?

Well, in the book I propose that human same sex attraction evolved specifically because it contributed to female sexual autonomy or to the freedom of choice. What I mean by that is that in the case of female/female sexual relationships they could contribute to female alliances that could protect females from sexual coercion by male hierarchical groups.

At the same time I propose that male/male sexual attraction could have evolved because any social situation in which males have multiple sexual outlets would have contributed to female freedom to move among individuals in that social system and to avoid coercion and sexual violence. This is a new aesthetic theory of the evolution of same-sex behavior in people, and I think it’s one that deserves really serious consideration as we move forward.You can read the book for a fuller explication.  The first paragraph explains lesbian relationships: they evolve because genes for lesbian same-sex attraction would help females bond and thus gain them female autonomy.  The problems with this are several. Why would male genes become unable to overcome the female “love bonding”? Why wouldn’t females just form coalitions without necessarily having it be a coalition based on sex? Bonobo females may show same-sex behavior, but female lions in prides don’t.

Further, if these genes are adaptive in our species, which is what Prum wants to explain, why haven’t they spread to fixation? That is, same-sex attraction in human females is not universal. (In fact, I think the evidence that it has a significant genetic component is not strong, though there is some evidence for a genetic basis of male homosexuality.) Yet it should be ubiquitous if Prum is right. So Prum’s explanation doesn’t explain why fewer than 100% of women are attracted to other women.

I suppose one could claim that the genetic influences have been overridden by culture, but that’s special pleading. Or one could claim that lesbian same-sex behavior is simply a side effect of selection for female “prosociality”, and when females are prosocial and bonding, a certain tail of that distribution will show lesbian behavior. But that’s not what Prum says: in the bolded bit above, he says that same-sex attraction was selected because it contributed to female sexual autonomy, not that same-sex attraction is a side effect of genes selected for sexual autonomy.

His argument for male/male sexual attraction is even weaker. Again, it fails to explain why fewer than 100% of males have homosexual tendencies; if the genes were useful (like the genes for female preference themselves, or for the male traits, like lion manes, which female favor), then why aren’t they seen acting in all individuals, just as Prum sees female preference and male traits? Again, given what Prum says, saying that some gay behavior is simply a side effect of females selecting for “nicer, gentler, and kinder males” who are noncoercive doesn’t fly.  Further, why wouldn’t male attraction to other males be at some disadvantage because males are wasting their reproductive effort in a way that doesn’t spread their genes? Prum may respond that females prefer to mate with homosexual males because they’re “noncoercive”, and that preference can outweigh the reproductive cost of gay males courting other males. But how this would get off the ground in an evolutionary sense defies me.

Until there are data supporting all of these ideas, Prum should do the responsible thing and keep his yap closed, or at the very least point out the problems with his own theories. That, in fact, is a recurring issue with the book: Prum doesn’t mention at all the problems with the “Beauty happens theory.”

But of course if he did that he wouldn’t get attention and book sales. In the process, however, he’s lost some of his credibility as a scientist. He’s not only in love with his Big Idea, but, like many scientists, appears to be enamored as well with the attention he gets. Take The Evolution of Beauty with a huge grain—nay, a big hill—of salt.

h/t: Tom