Scientific findings range from obvious to counterintuitive. New research published in the journal Evolutionary Behavioral Sciences undoubtedly exists at the counterintuitive end of the spectrum.
A team of evolutionary psychologists led by Gordon Gallup of the University at Albany, State University of New York found that women are more likely to fall asleep after sex than men. Their rationale? Because it increases the chances of conception.
“The phrase ‘sex is nature’s sleeping pill’ is often used to capture the idea that sexual intercourse may have sedative properties, but there has not been much research on this effect,” state the authors. “We found that women were more likely than men to report falling asleep after sexual intercourse and that post-copulatory somnolence was enhanced by orgasm in both women and men.”
While this finding may seem out of step with common sense, there's actually quite a bit of logic behind it. Here’s how the theory goes: Humans evolved an upright posture and bipedal movement as a means to navigate their environment in an optimally efficient way. One of the downsides of an upright posture is that it puts the female reproductive system at a downward angle with respect to gravity. This is not ideal for retaining sperm and, by extension, maximizing the chances of conception during sex. To overcome this problem, evolutionary psychologists note that “missionary position” has become a cross-cultural universal. But they argue there is another mechanism through which evolution has enhanced the odds of conception: by imbuing seminal fluid with sedative-like properties. This encourages women to remain lying down after sex, which allows more sperm to be retained in the reproductive tract and further increases the chances of conception.
To test their logic, the researchers recruited 316 undergraduates from the University at Albany to take part in a survey on sexual routines. They excluded individuals who indicated a non-heterosexual preference or who reported no prior sexual experience. They also excluded participants who indicated having more sex during daytime hours than in the evening given that the purpose of the study was to understand after-sex sleep routines. This resulted in a final sample size of 128 women and 98 men.
Survey participants were asked to fill out a questionnaire consisting of three sections: contraceptive use and relationship status, masturbatory behavior, and sexual history. Critical to their investigation were participants’ responses to the following three questions:
- Who typically falls asleep after penile-vaginal sex sooner, you or your partner?
- How often do you fall asleep after experiencing an orgasm during penile-vaginal sex?
- How often do you fall asleep after penile-vaginal sex without experiencing an orgasm?
Not surprisingly, both men and women reported being quicker to fall asleep after sex when experiencing an orgasm. However, with or without orgasm, women were more likely to report falling asleep sooner after sex than men.
Critical to their hypothesis was the finding that there were no gender differences in the sedative properties of masturbation. Only when women were inseminated did they report a greater likelihood of falling asleep after sex. This strengthens the argument that seminal fluid contains sedative-like properties.
This study is not without its limitations. For one, the sample size is relatively small and the data is correlational. In fact, the correlational nature of the study speaks to a broader critique of the field of evolutionary psychology, which is that many of its theories are virtually untestable (at least in the traditional sense).
The research also offers a window into the sexual behaviors of college-age adults. For example, for the question, "How often do you experience an orgasm during sexual intercourse," 50% of males reported experiencing an orgasm all of the time, compared to only 4% of females. Forty-nine percent of women reported experiencing an orgasm less than half the time, which was the most common response among women.
Furthermore, the data show that men masturbate more than women. Only 9% of men indicated that they do not masturbate at all in an average week while 4% of men indicated that they masturbate more than 10 times per week. For women, 45% indicated that they did not masturbate at all in an average week, 44% masturbated 1-2 times, 10% masturbated 3-5 times, and less than 1% masturbated 6-9 times.
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Gallup Jr, G. G., Platek, S. M., Ampel, B. C., & Towne, J. P. (2020). Sex differences in the sedative properties of heterosexual intercourse. Evolutionary Behavioral Sciences.