Some Fuss Over Sperm Competition: A Follow-Up


Back in March, I had discussed an argument put forth by Greg Cochran concerning his conclusion that sperm competition had no real impact on humans. For the sake of repetition, sperm competition is a context in which sperm from more than one male are present in a female’s reproductive tract during a period in which she might conceive; for humans, this often involves infidelity, but could also represent cases of non-committed double mating. In some sense, the sperm from different males can be thought of as different teams competing for the goal of fertilization of an egg. The metaphor isn’t perfect, but it should suit us well enough for the purposes of this discussion. In any case, in March’s post, I suggested the following:

Adaptations for sperm competition might be more subtle than larger testicles, for instance. Perhaps the frequency of sex – or at least the frequency and intensity of sexual interest – correlates with infidelity cues; perhaps the number of sperm per ejaculate could be varied facultatively as a function of sperm competition risk.

Clearly, I’m not the only one with such ideas, as a recent study I came across by Pham et al (2014) put the first suggestion – that the frequency of sex might correlate with sperm competition risk – to the test.

The paper begins by noting that the frequency of sex should contribute to a given male’s odds of winning this competition. The idea is pretty simple: more sex equals more viable sperm present in a female’s reproductive tract, and more viable sperm equals a higher probability of conception. It follows, then, that if a male perceives he is at a relatively high risk for sperm competition, we might expect his interest in engaging in sex with this partner to go up as a preemptive strike against non-paternity. The authors consider two factors which might correlate with the risk of sperm competition: a woman’s attractiveness and the number of opposite-sex friends or coworkers she has in her social circles. The former variable might play a role in that attractive women could be expected to draw proportionately more sexual attention or pursuit from males; the latter variable might matter because the larger the pool of males, the more overall interest a woman might receive. So, since more sexual interest equals more sperm competition risk, and since that risk can be mitigated by men by increasing the frequency of sex, we might expect that men should up-regulate their sexual interest in response to a perceived sperm competition risk.

To test this idea, Pham et al (2014) recruited approximately 400 men in committed relationships from a campus and community sample. These men were asked about their relationship length, their perceptions of their partner’s attractiveness, how many male friends and co-workers their partner had, and how often they had sex with their partner in the past week. Their analysis controlled for relationships length since, as almost anyone who has been in a relationship can attest to, the frequency of sex tends to decline over time. Predictable, this pattern also appeared in the results of the current study: men reported less sexual contact in the last week the longer they had been with their partner. There was also an effect of perceived partner attractiveness: the more attractive the men in the study thought their partner was, the more they report having sex in the last week as well. When it came to the risk of sperm competition – as indexed by the number of opposite sex individuals men perceived in their partner’s social circles – there was no effect on the frequency of sex. That is, whether the man reported their partners worked/were friends with few or many other men, there was no relationship to the frequency of sexual contact.

There was, however, a significant interaction between partner attractiveness and risk of sperm competition: when sperm competition risk was low, men with both high and low attractiveness partners reported the same frequency of sex in the last week: about 3 times. However, when the risk of sperm competition was high, men with attractive partners reported having sex about 4.5 times a week, whereas men with less attractive partners reported having sex about 1.5 times a week (though the absolute differences were about the same, only the former effect was significant). This would appear to be some evidence that is at least consistent with the idea that when men perceive there to be a higher risk of their partner having an affair with one or more other men (when other men are both present and interested), their interest in having sex with their partner increases.

There was, however, a significant interaction between partner attractiveness and risk of sperm competition: when sperm competition risk was low, men with both high and low attractiveness partners reported the same frequency of sex in the last week: about 3 times. However, when the risk of sperm competition was high, men with attractive partners reported having sex about 4.5 times a week, whereas men with less attractive partners reported having sex about 1.5 times a week (though the absolute differences were about the same, only the former effect was significant). This would appear to be some evidence that is at least consistent with the idea that when men perceive there to be a higher risk of their partner having an affair with one or more other men (when other men are both present and interested), their interest in having sex with their partner increases.

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One thought on “Some Fuss Over Sperm Competition: A Follow-Up

  1. I haven’t had the chance to read the study but it seems like there might be an obvious conclusion. Rather than “competing” against the risk of sperm competition, I think there’s an easier answer. If I find my partner more attractive… I’m going to want to have sex with her. If I find my partner less attractive… I’m going to be less interested in sex. It might not have anything to do with competition. Maybe they addressed that point in the post but it seems like the more obvious answer to me.

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