Doctor Science Knows

Saturday, September 08, 2007

Reporting on Sexual Selection

A lot of people have linked to the CNN report about a study of human mate preferences with the headline "Men want hot women", which hilzoy called the Dumbest Headline Ever. What seems to have slipped by is that CNN's headline isn't just dumb, it's wrong -- that's not what the study found.

I haven't seen the full study (subscription required), but the press release helps me decode it a bit, as does the abstract. The article, "Different cognitive processes underlie human mate choices and mate preferences", is by Peter Todd et al., an international group mostly at the Max Planck Inst. in Berlin.[1]

As a biological rule, sexual selection is about female choice. Males do not, generally speaking, choose mates, they get chosen by females.[2] In the vast majority of animal species, males fight each other to be the one chosen by females. They compete for territories so females who are basically house-hunting will choose them along with the house. They are the ones who grow the bright colors or the elaborate antlers as conspicuous biological consumption, to show they have health and vigor to burn.[3]

There are a few species of birds where sexual selection is reversed and brightly-colored females compete for drab males, but this is never the case for mammals. When I was in grad school (in evolutionary biology) it is a byward that "milk is the limiting factor for mammal populations", which means that female mammals have even more at stake than female birds and are correspondingly choosier.

In other words, any time you see a popular discussion of human evolutionary biology, psychology, or sexual selection that does not focus on female choice, something fishy is going on. By "fishy" I mean:

a) the person doesn't actually understand evolutionary biology and is just making stuff up to fit their (his) prejudices and presuppositions


b) they are talking about something that is not based in biology, but *is* culture


c) they are talking about a way human biology differs from that of other animals, a way humans are distinctive and even abnormal

It's clear to me that Todd et al. are doing none of these things. This is really a methodological paper, to establish that speed dating can be used to study human mate choice. Speed dating is superficial and limited, but it also lets researchers study a lot of human interactions very quickly and without many ethical difficulties. This is a preliminary, does-this-fly study, that Todd et al. hope to use as the basis for studies with larger sample sizes.

One reason Todd et al. think speed dating is a reasonable thing to study is that they found men to be much less choosy than women. Again, this is exactly what an evolutionary biologist would predict: that female choice is crucial, while males will basically take whatever they can get. I think that if Todd et al. had found men being pickier than women at speed dating they would probably have thrown the results out as being culturally determined and therefore, from an evolutionary perspective, uninteresting.

This is what they mean by writing in the abstract that
Unlike the cognitive processes that Buston and Emlen inferred from self-reports, this pattern of results from actual mate choices is very much in line with the evolutionary predictions of parental investment theory.

Now, I think they're wrong. I don't think speed dating is a good predictor of human mate choice, whether "mate" means "person you live with" or "the other parent of your offspring". I think the anthropological evidence is that human mate choice (however defined) is *both* (a) subject to strong cultural influence, and (b) biologically distinctive.[4] I also see plenty of evidence that in most cultures sexual selection on females is as strong as sexual selection on males. This is a biologically unusual pattern, and thus should have biologically unusual causes and produce unusual results.

The mere fact that Todd et al. are using "speed dating" and "parental investment theory" in the same place should make you think twice -- or LOL, because becoming parents together is not really what the participants have in mind. More seriously, parental investment theory doesn't predict that all mating systems should be the same, or that females should *always* be doing the sexual selection -- it depends on the organism's ecology and reproductive biology.[5]

But, whether you agree with Todd or not, you should notice that CNN's headline says nothing at all about women's choices and how important they are: it's just What Men Want. Is this is common-or-garden variety sexism (in which men *must* be the active party), is it that the CNN editors subconsciously realize that women are choosy and thus need more information, is it part of a pervasive problem with science journalism echidne has discussed, is it just stupidity? I personally am splitting my vote between "sexism" and "stupidity", but you be the judge.

[1] PNAS, the Proceedings of the National Academy of Sciences, has a complex and idiosyncratic submission policy which used to have a very large component of pure ego- and nepo-tism. That has been reduced, but it's still important to note who is listed as the "Editor" of a particular article. This one is credited to Gordian Orians, a evolutionary biologist (expert on vertebrate mating systems) for whom I have huge respect but who's not all that young anymore.

[2] This article is a good, non-controversial overview of sexual selection and mate choice.

[3] For instance, it's male peacocks who have the gorgeous tails, male deer who have antlers, male lions who have manes. Do I need more examples, or does everybody know this?

[4] Notably, in the vast majority of human cultures people other than the couple have input into human mate choice. This has no parallel in nature, which means it should have effects that a naturalist would not expect.

[5] For instance, marmosets and tamarins are small South American primates that are frequently polyandrous, where a single female mates with more than one male. Male marmosets do a great deal of baby-carrying and other child-care (=high paternal investment). Probably because of the extra help from the males and from older siblings, marmosets usually have twins and occasionally even triplets, while twinning is rare in other primates.

You can see from the abstract of this review article ("Ontogenetic variation in small-bodied New World primates: implications for patterns of reproduction and infant care") that parental investment theory doesn't predict a single social system for even this one group of creatures. It depends on what they eat, how fast the offspring grow before and after they're born, how large a group of adults can find food together, and even on twins being genetic blended in the womb. (that last article is *boggling* -- germ-line chimerism! in a primate! no wonder they're almost eusocial. I'm gobsmacked, scientifically speaking. I'll write more about this later.)

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