Keeping It Casual, Part 3
The evolution of sex differences in sexual psychology: It’s not (just) what you think
Why are men more interested than women in casual sex and sexual variety?
The standard answer is simple: Men can father more children, so they evolved to want more partners.
It’s a nice, tidy story - and it’s partly true.
But it also fails to explain a lot of what we actually see in the world. In many species, females are far more promiscuous than the standard story predicts. And in our own species, sex differences in sexual psychology, though real, are smaller than you might expect.
In this excerpt from my forthcoming book A Billion Years of Sex Differences, I lay out a new way of thinking about the evolution of sex differences in sexual psychology - one that goes beyond the usual focus on maximum offspring number and brings a wider range of evolutionary forces into view.
You can access the full collection of excerpts here, and preorder the book here (UK) or here (US).
The Evolution of Sex Differences in Sexual Psychology: It’s Not (Just) What You Think
We saw in the last excerpt that sex differences in sexual psychology aren’t just a product of culture; there’s also an innate contribution. Importantly, though, to say that there’s an innate contribution is to say nothing about how it came about. Why did natural selection ‘decide’ to give men and women somewhat different, somewhat incompatible desires? If a god were behind it, we might suspect that he was playing a cruel joke on us or trying to make the soap opera of human life more entertaining for himself. With natural selection as the architect, however, no such explanation is available. Somehow, male-typical sexual leanings must have been adaptive for our male ancestors, whereas female-typical sexual leanings must have been adaptive for our female ones. The only real question is why.
Now, you might think you know the answer already. I touched on it in Chapter 4, and in any case, the basics are quite well understood these days. But not so fast. As with sexual selection, the evolutionary explanation for sex differences in sexual inclinations is long overdue for an overhaul. The aim of this section is to sketch out a new and improved version of the explanation. We’ll start with the standard story, then gradually thicken the plot.
The standard story goes like this. As a result of sex differences in reproductive physiology, men can potentially have many more offspring than women. A man who has, say, five sexual partners in a year could end up with five new offspring; in contrast, a woman who has five sexual partners in a year would probably end up with no more offspring than she would if she’d just had one.
This created very different selection pressures for our male vs our female ancestors. Men who sought multiple partners typically had more offspring than men with less wide-ranging interests, and the genes underpinning that tendency became increasingly common in the human gene pool. Men evolved a higher sex drive, a greater hunger for casual sex, a stronger desire for sexual variety, and a capacity to be quickly aroused by sexual stimuli (which for most of our evolution meant flesh-and-blood women rather than porn).