Evolutionary psychology is known for its unique and sometimes controversial view of human behavior. Why do men prefer symmetrical facial features in women? An evolutionary psychologist might say it’s because it signals ‘high reproductive potential.’ Why is it that middle-aged women have the stereotype of being more sexually interested than younger women? Evolutionary psychology suggests that increases in sexual interest should coincide with decreases in fertility.
New research tackles another mind-bending evolutionary phenomenon: why women are more likely to fall asleep after sex than men. The answer? Because it increases the chances of conception, at least according to a new paper published in the journal Evolutionary Behavioral Sciences.
“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 of the paper, led by Gordon Gallup of University at Albany, State University of New York. “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 counterintuitive, here’s a step-by-step overview of the rationale behind it:
- Humans evolved an upright posture and bipedal movement as a means to navigate their environment in an optimal way.
- One of the downsides of an upright posture is that it puts the female reproductive system at a perpendicular orientation 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 point out that “missionary position” has become a cross-cultural universal.
- But they argue there is another mechanism through which evolution has stacked the deck in favor 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 female reproductive tract and further increases the chances of conception.
To test their reasoning, the researchers recruited 316 undergraduate students from the University at Albany to take part in a short survey on sexual routines. They excluded individuals who indicated a bisexual or homosexual preference or who reported no prior sexual experience. They also excluded participants who indicated having more sex during the day than at night given that the purpose of the study was to understand post-coital sleep routines. This resulted in a final sample of 128 women and 98 men.
Participants were asked to fill out a survey consisting of three sections: contraceptive use and relationship status, masturbatory behavior, and sexual intercourse 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 were 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.
Crucial 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 bolsters the argument that seminal fluid may contain sedative-like properties.
This study is not without its limitations. For one, the sample size is relatively small and the data is correlational, not causal. In fact, the correlational nature of the study speaks to a broader criticism of the field of evolutionary psychology, which is that many of its theories are virtually untestable (at least in the traditional sense).
That said, most scientists know better than to bet against theories of evolution.