Cordelia Fine on differences in promiscuity between men and women

Cordelia Fine just published her latest book, which has received extremely positive reviews in various newspapers. I haven’t read it yet, but judging from what the reviews say, I suspect it’s not as good as the reviewers think. For instance, here is what Annie Murphy Paul, who wrote the book’s review in the New York Times, says about how Fine attempts to debunk the claim that men have evolved to be more promiscuous than women:

Well, then, what about the even more entrenched idea that evolution has primed men to desire many and varied sex partners? Here Fine quotes the Bradley University psychologist David Schmitt: “Consider that one man can produce as many as 100 offspring by indiscriminately mating with 100 women in a given year, whereas a man who is monogamous will tend to have only one child with his partner during that same time period.” Fine expertly fillets this familiar premise, noting, among other inconvenient facts, that “the probability of a woman becoming pregnant from a single randomly timed act of intercourse is about 3 percent,” and that in historical and traditional societies, as many as 80 to 90 percent of women of reproductive age at any one time might already be pregnant, or infertile while they were breast-feeding. “The theoretical possibility that a male could produce dozens of offspring if he mated with dozens of females is of little consequence if, in reality, there are few females available to fertilize,” Fine comments. Think about it: For every man on the prowl, there simply aren’t a hundred women available to bear his child. For all men not named Genghis Khan, monogamy must have started to look like a pretty smart bet.

I wasn’t sure that Paul had not misrepresented Fine’s argument, so I read the relevant part of the book, which confirmed that she had not. Fine spends about 1/3 of the chapter in question arguing that, despite what Schmitt says, the probability that a man would produce 100 children by mating indiscriminately with 100 different women in a given year is infinitesimal.

The argument she criticizes is that, since a man can produce more children by having intercourse often with several different women, whereas that is not true of a woman, promiscuity is evolutionary advantageous for men but not for women and, therefore, you would expect men to be genetically predisposed to be more promiscuous than women on purely theoretical grounds. To be clear, there are various things which could go wrong with that argument, but I don’t want to get into that in this post. (For instance, it could be that, in environment of evolutionary adaptation, sticking with a woman after they had impregnated them was actually a better strategy for men, because otherwise their children had a much lower probability of survival.) I don’t even want to argue that men really are genetically predisposed to be more promiscuous than women, although for various reasons I think it’s extremely plausible. Instead, I just want to explain why Fine’s so-called debunking of the argument I just presented does not actually debunk anything, which is a much more modest claim.

Indeed, it only takes a moment’s reflexion to see that the probability that a man would produce 100 children by indiscriminately mating with 100 women in a given year is extremely low, but that’s clearly irrelevant to the argument Fine claims to debunk. As long as it’s true that a man can have more children by mating with several different women, whereas a woman cannot, which at least prima facie seems to be true, you would expect men to have been selected for promiscuity. As anyone with a basic knowledge of quantitative genetics knows, even a numerically small selective advantage can drive the frequency of a gene up in a population if you wait long enough, for essentially the same reason that a numerically small interest rate can easily result in very large amounts of money when it’s compounded. So it doesn’t have to be the case that the probability that a man would produce 100 children by indiscriminately mating with 100 women in a given year is very high for the argument to go through, it just has to be the case that a tendency to be promiscuous increases the mean number of children — even just a little bit — for men but not for women.

This example illustrates a common problem with Fine. It’s not that everything she says is false, because she frequently makes good points, but she often goes after the weakest targets or interpret the people she criticizes in the most uncharitable way possible, as she does with Schmitt in the example I just discussed. She is right that scientists often engage in fallacious reasoning when they argue that such and such a psychological difference between men and women is genetic, but she seems to be convinced that no psychological differences between men and women are genetic, which is absurd. Of course, she never says that, but the fact that she often doesn’t address the best arguments/evidence in favor the view that some psychological differences between men and women are genetic suggests that, for any psychological trait that seems not to be distributed identically among men and women, her priors seem to be irrationally stacked against the hypothesis that it’s at least in part for a genetic reason. After all, many behavioral/psychological differences between the sexes exist in virtually every animal species, so it would be truly amazing if humans were the only exception, even if culture makes everything more complicated.

Unfortunately, many people will be fooled by her book, even some who should know better. (I may or may not be talking about philosophers, who usually don’t know much about biology.) You may think that it’s not that bad, since it will only correct the opposite tendency to think that every psychological difference between men and women is genetic, but I think it’s unlikely. Indeed, while this opposite tendency is also very common, people who have it probably won’t read Fine’s book. Instead, I’m afraid it will only strengthen the prejudices of people who irrationally believe that, to the extent that men and women are psychologically different, it has nothing do with genetics, because it’s mostly them who are going to read her book. Which is why it would be nice if prominent newspapers such as The New York Time asked people who actually have some understanding of science to review that kind of books.

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