Most women probably don't make a connection between how many male friends they have and how much sex they're having in their relationship. But a new study suggests that for evolutionary reasons, the former may in fact have a direct influence on the latter.
The research, which was recently published in the Journal of Comparative Psychology, suggests that a man who considers his partner attractive to others is likely to try to have more sex with his partner if he perceives that she has other males in her life.
Whether they know it or not, men perceive other males in their partner's life as sexual threats.
This is a result of what evolutionary psychologists call sperm competition, which simply means that a man will try harder to inseminate his partner if he thinks he has competition from potential sexual rivals. These men may be feeling romantic jealousy when they draw closer to their partners, but there's also at least one other force at work -- a subconscious desire to ensure that their sperm produces the most offspring.
For the study, 393 men in committed, sexually active, heterosexual relationships were recruited. Participants rated how sexually and physically attractive they found their partner, as well as how sexually and physically attractive they thought other men found their partner. They then listed how many male friends and co-workers their partner had, before reporting how many times they'd had sex with their partner in the past week.
As expected, the researchers found that couples had more sex when the woman had greater interaction with other men. This suggests that the number of sexual rivals in a woman's environment can cue sperm competition in her male partner, motivating him to initiate sex with her more frequently.
It's worth noting that this sample showed a skewed distribution, where, on average, men thought their partners were physically and sexually attractive to others as well as themselves. The study was also limited in that the researchers didn't know which partner actually initiated sex -- they just knew that these couples had sex more frequently. (Female-initiated sex, obviously, isn't a male sperm competition tactic.)
This behavior isn't unique to human males -- many animals do the same thing.
"The reason we specifically predicted this is because there's a huge amount of nonhuman literature showing that male animals, for example mice or rats, become very interested in having sex with their partner when they see their partner interacting with other males," Todd K. Shackelford, professor and chair of psychology at Oakland University and a co-author of the study, told The Huffington Post. "They don't even have to see them having sex with other males. It's just the presence."
Even though modern birth control methods have largely decoupled sex from reproduction, Shackelford said it doesn't matter whether sexually jealous men actually produce more babies. The point, he said, is that modern humans evolved this way, and the psychological response still appears to be active in men today.
According to Shackelford, it's highly unlikely men even know they're acting out of Darwinian jealousy when they initiate sex as a result of sperm competition. As he noted, this pattern of behavior is observed in various species, from mice to birds to insects. Since the competitive urge presumably makes itself felt at the brain's lower levels for these animals, it likely works the same way in humans.
While this knowledge may not help you navigate a romantic relationship, it's still useful.
That doesn't mean there aren't pragmatic takeaways from these findings, particularly in the field of medical fertility. Shackelford said that, since men provide semen for use in human fertility treatments, medical professionals can capitalize on the high-quality ejaculate that's produced under conditions of sperm competition.
The example Shackelford gave had to do with the porn men could watch when they provide semen samples. If you ask one man to provide a masturbatory ejaculate while watching a scene of multiple men having sex with one woman, and ask another man to do it while watching a scene of one man having sex with multiple women, you'll find that the quality of the ejaculate is higher in man who observed the sperm competition environment, i.e. the former scenario.
Perhaps most importantly, Shackelford said that his findings about human male sperm competition could provide insight into instances of sexual violence and nonconsensual sex. In 2005, he co-authored a study whose findings suggested a correlation between a man being sexually coercive in the context of a long-term romantic relationship and the degree to which the man perceived his partner to have been unfaithful to him.
Those findings, in turn, speak to Shackelford's more recent work suggesting that the more males a woman is friends with, the more sexual rivals her partner perceives himself to have.
Shackelford acknowledged this as a limitation of his more recent study, since the sex these couples were having could very well have been, as he put it, "part of a broader anti-cuckoldry strategy" on the part of the males. In other words, a man who's worried about sexual rivals might consciously make the decision to initiate sex more often, to make sure his partner is sexually satisfied and won't look outside the relationship. It's not clear, for now, to what extent the couples in Shackelford's study were having more frequent sex because the men were making a conscious choice to initiate more, and to what extent the men were simply responding to subconscious biological imperatives. (And again, it's not even clear how often the men in these couples were the ones initiating sex.)
As for the small-scale, personal application of these findings for couples? Shackelford said that aside from people possibly gaining a better understanding of their partner's sexual reactions, he's not quite sure how being aware of this behavior might affect healthy relationships.