This paper on men falling asleep after sex is Exhibit A for weak evolutionary psychology

A new study purports to use evolutionary psychology to explain why men fall asleep after sex, and what it actually means. The study is being widely reported as proving that men only fall asleep after sex to avoid giving affection or commitment to their female partners. Or maybe it proves that men who fall asleep after sex really are in love, after all.

So... what does the study actually prove? Nothing. It's a flawed study that proves only that people are eager for "findings" that prop up gender stereotypes.

Top image: Olly/Shutterstock.com

Researchers Daniel Kruger of the University of Michigan and Susan Hughes of Albright College just published this new study, in which they examine what they call the Post-Coital Time Interval, or PCTI. Their study, which appears in the Journal of Social, Evolutionary, and Cultural Psychology, asked 456 participants - 295 female and 161 male undergraduates from a pair of Midwestern universities - to complete an anonymous online survey where they detailed what they wanted from the PCTI experience, as well as which partner was more likely to fall asleep first after sex.

The basic findings were these: whatever the stereotypes might be, men were no more likely to fall asleep before women after sex, according to the respondents. However, the researchers found that men were more likely to stay awake longer if sex hadn't taken place. Also, those whose partners fell asleep shortly after sex were the ones most likely to express a desire for cuddling and talk during the PCTI.

These are some interesting results, but dig a little deeper and you start to see some problems with the study. For starters, of those 456 participants, 181 of them were eliminated from the study - 163 who "reported not yet having full sexual intercourse", and 18 who said they were equally or primarily attracted to partners of the same gender. So the results here purposely only apply to heterosexual couples, and the question then is how much these results can be filtered through the prism of evolutionary psychology, in which all these post-coital activities are artifacts of humanity's ancient behaviors.

The original paper is fascinating reading in this regard, particularly a lengthy background piece that runs through all the prior studies on the differences in male and female reproductive strategies. It's comprehensive, well-researched background — but it's also reliant on the assumption that most modern behavior is explained by evolved reproductive strategies, in which woman are guided by finding a long-term male partner who will provide resources to rear children, while men want to find as many partners as possible to maximize their reproductive success.

That assumption leads to passages like this, in which the researchers consider why males would stay up longer than females if sex hasn't taken place:

Perhaps men may stay awake longer as an artifact of mate guarding, ensuring that their partner does not secretly leave them for another man. Men may also remain awake longer in attempts to entice their partners to engage in sex. As the costs and benefits of sexual acts are different for men and women, women may have an incentive to fall asleep earlier to decrease the likelihood of sexual acts in most circumstances.

I don't think the researchers are suggesting here that men actually stay awake longer because they are afraid their partner will leave for another mate. Instead, I'm pretty sure they're suggesting that men stay awake longer because ancient humans were afraid their partner would leave for someone else. I'm not even sure which of those is the shakier suggestion.

Here's another representative passage, this time throwing around some big and frequently rather bizarre ideas on why one partner might fall asleep before the other:

Falling asleep before one's partner may be a non-conscious mechanism that forecloses on any commitment conversation occurring after sexual intercourse. Because women have greater verbal ability on average (Halpern, 1992) and greater density of neurons in brain areas associated with language processing (Witelson, Glezer, & Kigar, 1995), they may have an advantage in these commitment negotiations. If men actively avoid commitment promises in post-coital conversation, this could increase the likelihood of women ending the relationship due to perceptions of undesirable partner characteristics and/or uncertainty about the future of the relationship. Hastening sleep onset may evade this adverse effect.

The whole paper is well worth reading, as it's a perfect encapsulation of why we're often so dubious about evolutionary psychology around here. The basic experiment has provided some interesting data, and the paper cites tons of previous sources that help build up a picture of the larger psychological mechanisms that might be at work here. This all feels like good science, and yet somehow it all seems a bit...off.

The reason for this, I'd say, is because the two strands of evidence don't actually seem to link up into a cohesive whole unless you're willing to make the leap of faith that all our behaviors are molded and motivated by these deep-seated, highly evolved reproductive strategies. But if you can't make that leap - and I'd very much include myself in that number - then the whole thing sounds pretty damn ridiculous.

Read the original paper here. Top image by olly, via Shutterstock.