Even after controlling for variables like age, religiosity, and political affiliation, the study authors found that people who saw female financial dependence on men as more common were also more likely to negatively judge promiscuity in both sexes.

One possible reason for this correlation boils down to some pretty old-school reproductive math:

In environments in which female economic dependence on a mate is higher, both a woman and her mate have a greater interest in maximizing paternity certainty. Because promiscuity undermines paternity certainty, both men and women should be more opposed to promiscuity by both sexes.

We’ve evolved to consider sex, the researchers argue, as a game of finite resources. For our ancestors, multiple sexual partners meant things could get knotty when it came to proving whose kids were whose. For women who depended on men for their livelihoods (and the livelihoods of their offspring), that uncertainty meant losing out on the support of their male partners. Bad news. For men, it meant investing in the well-being of children they hadn’t necessarily fathered. Also bad news.

The connection between sexual behavior and morality, then, may have come about as a way of keeping a gender-based social order intact. “Through moralizing,” the researchers wrote, “individuals can promote behavior which serves their own personal and coalitional interests.” Back in the day, judgment was a form of defense.

While religious arguments against casual sex still exist, the paternity justification for promiscuity’s immorality is of another time. Sex and pregnancy no longer have to be synonymous if we don’t want them to be (and most don’t—more than 99 percent of sexually active women in the U.S. have used birth control at some point in their lives, according to the Guttmacher Institute). Paternity tests exist. The idea that a man should forever be his family’s sole breadwinner seems more than a little anachronistic. The idea of family itself is changing in ever-expanding ways.

But when it comes to this particular area, we don’t really care. As the Archives of Sexual Behavior paper explains, “The beliefs may persist due to evolutionary adaptive lag, that is, because the environment has changed faster than the moral system.” In other words, our psyches are sluggish—and in a rapidly evolving world, they haven’t necessarily kept pace.

Although the stakes may have changed, sexual judgment remains a form of self-preservation. The Archives of Sexual Behavior study isn’t the first to posit that women, in particular, are hardwired to slut-shame out of self-interest (among the judgers, women were more opposed to casual sex than men). As Olga Khazan wrote in The Atlantic last year:

In his book, The Evolution of Desire: Strategies of Human Mating, Buss argues that women do this because, evolutionarily, women who are willing to have casual sex undermine the goals of women who want long-term relationships. “Slutty” women hint to men that it’s okay not to commit because there will always be someone available to give away the milk for free, as it were. Their peers’ “derogation” is thus intended to damage the reputation of these free-wheeling females.

Even now, so ingrained is the instinct to judge, it seems, that we do it even when our morals and our actions aren’t exactly congruous. A third of Americans told Gallup last month that they don’t believe in premarital sex—even though, as Emma Green recently noted, 95 percent of Americans were doing it as of 2006.

So, again: Is casual sex immoral? The answer may change with place and time, but judging other people’s sex lives remains an act as innately human as sex itself.

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