Note: By invitation, I’ve started a blog on the Psychology Today website, and my latest post can be read there or here, although different (and likely more numerous) comments will be  posted there. This entry resembles and updates one I posted here in March 2009, which was followed by an interesting exchange on “insatiable widows” and other cross-cultural myths.

We hear a lot about sex differences, and arguments rage over which are real. Evolutionary theorists weigh in about why this or that difference should be expected, while some anthropologists say cultures vary so much that generalizations are folly. But of all Darwinian predictions about la différence, few are as logical as the one about sex differences in sexuality. Here’s why.

Women risk so much more in any coupling that natural selection should have made them wary; those who were not took reckless risks. Males have a lot less to lose, so women should be more reluctant and choosy. Or, as anthropologist Donald Symons said, most cultures have understood that sex is something that women have and men want. An exaggeration, certainly. Women who didn’t want it at all, ever, didn’t leave many offspring. But between the sexes all is relative, and women have hand. Their reproductive power sets a limit, their big investment is at stake every time, and a uterus is a hugely valuable thing.

In the anthropological record about two thirds of societies made men offer wealth or service to their brides’ families; this “brideprice” had no exact counterpart on the other side-dowries, practiced in about three percent of cultures, were traditionally from the bride’s family to the couple, and occurred in highly stratified societies where there was severe competition for the highest-status men.

But neither evolutionary theory nor cross-cultural data can prove a sex difference. Here’s where psychological studies come in, and two reviews by Roy F. Baumeister and his colleagues are relevant. The first, co-authored by Kathleen Catanese and Kathleen Vohs, asked, “Is there a gender difference in sex drive?” and reviewed hundreds of studies. The answer is an unequivocal yes.

Men think about sex more often than women, experience sexual arousal twice as often, have more frequent and varied sexual fantasies about more different partners, masturbate much more frequently, want more partners in the future, and expect sex earlier in a relationship-the actual timing being highly correlated with women’s expectations, not at all with men’s. Virtually all sexual practices, “normal” or otherwise, are more likely to be desired by men.

Women tend to be satisfied with the amount of sex in their marriages but have husbands who want considerably more, even after many years together. Men are twice as likely to initiate sex. Complaints of low sexual desire and difficulty achieving orgasm are far more common in women than men, and the most common sexual problem in young men is premature ejaculation, a sort of excessive eagerness. Or if you invoke “nervousness,” then it often seems to have opposite effects on the two sexes.

Now take the “opposite” sex out of the picture. That should clarify things, especially in a world of male dominance and coercion. Well, gay men have far more sex partners than lesbians, and are more likely to cheat on a long-term partner and less likely to abstain for long periods. Among Catholic clergy, women are more successful in achieving and maintaining celibacy than men.

Now-classic research by Russell Clark III and Elaine Hatfield was published in 1989 after a decade of rejections. Women and men of average attractiveness approached college students of the other sex and asked if they would go to bed with them; 77 percent of the men said yes, 100 percent of the women said no. Two recent replication studies in different countries had less extreme results: women said yes 6 percent and 4 percent of the time (the first study was of women only; the second had a male “yes” rate of 45 percent). Sex with a stranger is the ultimate evolutionary litmus test, and the difference is huge.

In 2004 Baumeister and Vohs looked at sexual economics, with few surprises. Female sex workers routinely charge men for sex; their clients are almost never women. Men sell sex, but-despite the dramatic exceptions-their clients are almost all male. In heterosexual relationships, women speak of “giving” their virginity as a gift, men do not.

What of those who can have a surfeit? Attractive female sports stars resent and fend off sexual attention; males tend to welcome it and take advantage, up to thousands of partners. Even in our liberated culture, men tend to pay for dates during courtship, and there is no “gentlemen’s night” at the local club where women pay the cover charge and men get in for free.

Darwinian predictions may be neat, but theory doesn’t make them true; only the long slog of research can do that. In this case, Symons was basically right. He also noted that for a spell in the sixties and seventies men almost had women convinced that they had as much to gain in sex as any man.

Luckily for our daughters, this trick worked only transiently, and neither sexual revolution, contraception, nor women’s growing economic power appear to have changed the underlying reality.