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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 (thụ thai).

“The phrase ‘sex is nature’s sleeping pill’ (tình dục là thuốc ngủ của thiên nhiên) is often used to capture the idea that sexual intercourse (quan hệ tình dục) may have sedative (giảm đau, an thần) 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 (sau giao cấu) somnolence (buồn ngủ, lơ mơ, mơ màng) was enhanced by orgasm (cực khoái) 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 (hệ sinh sản) at a downward angle with respect to gravity. This is not ideal for retaining sperm (tinh trùng) and, by extension, maximizing the chances of conception (thụ thai) during sex. To overcome this problem, evolutionary psychologists note that “missionary position” (làm tình kiểu truyền giáo, truyền thống) 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.

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