A recent study in this area looked at a new aspect of the theory, studying a phenomenon seen in nonhuman animals, in which the amount of a female mate receives from other males predicts the number of times her male mate will have sex with that female. In other words, it’s been observed that male primates get jealous when their female mate gets lots of attention, grooming, etc., from other males. When this happens, the male mate is more likely to have lots more sex with the female, in a behavior believed to be an expression of sperm competition. By having more sex, the male mate is increasing the chances that his sperm will fertilize the female, as opposed to the possible sperm of these other presumed suitors.

In humans, it’s of course more complex: Most human relationships and environments don’t involve easily observed sex, for one thing. Further, in today’s world, we commonly interact on a daily basis with far, far more people, of both sexes, than humans interacted with in the early days of our evolutionary history.

The authors of this study relied on self-report strategies for data from almost 400 men in committed, heterosexual, monogamous relationships. The men reported not only how much sex they had with their female partner, but also how many male friends and coworkers their partner had, and how much attention they thought their partner got from those men. Finally, in a clever twist, the guys also ranked how attractive they thought their female partner was, or at least, how attractive they thought those other guys would think she was.

The point of this last measure of was to determine whether the degree to which men think their girlfriend/wife is seen as a “catch” by other men affects how much they feel threatened when their partner gets lots of attention from those other guys. If a guy doesn’t think other men are likely to find her that attractive, will he still be worried by the idea that they are paying lots of attention to her?

The researchers found strong evidence that, in fact, the number of male friends and coworkers a woman had did predict how much sex the woman had with their male mate—but only when the man believed that she was likely to be seen as “hot” and desirable to those other men. Whether those other males were friends or coworkers didn’t seem to make a difference; the females' partners judged those other men as threats whether they were social or professional relationships. Given the wealth of evidence about how many sexual relationships start in the workplace, we can’t say the men are misguided in their concerns. (I do wish they’d also looked at social media friendships, as they reflect a new, and often challenging, aspect of modern relationships.)

I have one rather substantial quibble with this study: It looks exclusively at the males’ self-report. Granted, the man’s self-report is a strong reflection of his perceptions, and it is his way of seeing things that is most likely to affect his behavior, if sperm competition is really at root of this issue.

But, what about the women? The authors ignore a very, very strong variable here, which is likely to confound these results: Female libido. The higher a woman’s sexual libido, the more likely she is to have more male friends. Not because she is having sex with them, but because women with higher sexual desire get along better with other men, and often, are seen to act more like guys. Other women may try to “slut- ” women with high libido, whereas some men may simply view it as fun, exciting, and like themselves.

Men whose female partner has higher libido are likely to have more sex, on average—reflecting her interest in sex. And those men are more likely to view their partners as being more attractive to other men, because they know that other guys desire highly sexual women. Further, the male mates of high-libido females are, logically, likely to have significantly more concern about their mates engaging in , or being tempted by it, compared to women with lower libido.

These variables don’t have to be independent, but likely interact in a synergistic manner in which female libido may enhance sperm competition behaviors. Ignoring the issues of female desire is a common failing of much evolutionary psychology research, which sadly contributes to the perception that evolutionary psychology can be somewhat misogynistic, or at least sexist.

Are women with more male friends more likely to cheat? Probably not, at least not based on that single variable. is a complex, over-determined behavior affected by many variables and environmental issues, which all interact. But concerns that other men may be trying to “poach” one’s attractive, sexy female partner are very real for many men; I know that I hear from many guys who worry over their female partners’ “guy friends” who might get too close.

But men can take this and concern and turn it on its head, viewing it all as a compliment to themselves. If they are fortunate enough to have a female partner that other men want to be around, that says something about their own value. Like the song says:

You can dance, go and carry on Till the night is gone and it's time to go If he asks, if you're all alone Can he take you home, you must tell him, no 'Cause don't forget who's taking you home And in whose arms you're gonna be So darlin', save the last dance for me...

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