Scientists have found that when they put a male rat in the company of a female rat in heat, he becomes the mating equivalent of the Energizer bunny—he keeps going and going (or perhaps more accurately, he keeps coming and coming, given that rats can ejaculate several times in short succession). After mating until exhaustion, he may appear to be completely spent; however, he will perk up and start going at it again if introduced to a new female.
This phenomenon, in which a new partner seems to awaken sexual interest, is known as the Coolidge Effect, which received its name from a popular anecdote about former President Calvin Coolidge and his wife. As described in the classic textbook A New Look at Love, the story goes like this:
“Mrs. Coolidge, observing the vigor with which one particularly prominent rooster covered hen after hen, asked the guide to make certain that the President took note of the rooster’s behavior. When President Coolidge got to the hen yard, the rooster was pointed out and his exploits recounted by the guide, who added that Mrs. Coolidge had requested that the President be made aware of the rooster’s prowess. The president reflected for a moment and replied, ‘Tell Mrs. Coolidge that there is more than one hen.’”
To date, most research on the Coolidge Effect has been conducted on animals, with studies finding that it occurs to some extent in both male and female rodents, among other species (e.g., insects). Evidence for the Coolidge Effect in humans is relatively limited (a human study that is equivalent to the rat scenario described above is challenging to pull off, ethically); however, a new study published in the journal Evolutionary Psychological Science provides support for the idea that the Coolidge Effect does exist in humans and suggests that it may have evolutionary significance.
In this study, 23 young heterosexual guys watched some porn and donated their semen to science (it’s a tough job, but someone has to do it). Over a 15-day period, these guys watched seven different porn clips, each spaced by 48-72 hours. The first six clips featured the same male and female performers having sex, with each clip drawn from different segments of one longer film. The seventh and final porn clip featured the same male performer from the first six clips having sex with a different female partner, “who differed distinctly from the first woman in facial and body features, hair color, and tattoos.”
Participants masturbated to each film clip in a private room and used a “wide-mouth” collection cup to capture their ejaculate for later analysis. What the scientists wanted to determine was whether the timing, amount and quality of semen produced changed when a new female was introduced after men had masturbated to the same female repeatedly.
When scientists compared ejaculate parameters from the sixth trial (which featured the same woman they had seen several times before) to the seventh trial (which featured the new and distinctly different woman), several statistically significant differences emerged.
First, men ejaculated faster when they saw a new woman—specifically, they climaxed a little over eight seconds earlier on average. Not only that, but these quicker ejaculations included a greater volume of semen in which there was a larger number of motile (i.e., active) sperm.
Exposure to a novel woman was thus not only linked to increased arousal (as evidenced by faster ejaculation) but also to changes in the nature of the semen produced.
How do we explain this pattern of results?
According to the authors, they suggest “selective pressures in our evolutionary past for greater investment in and more rapid transfer of ejaculates in matings with females other than a male’s social partner (i.e., extra-pair copulations).”
In other words, they propose that men might have evolved this way in order to increase the odds of reproductive success when they cheat. Specifically, climaxing faster would theoretically reduce the risk of being caught in the act, whereas releasing more active sperm would induce sperm competition (i.e., it would make his “swimmers” more competitive for fertilization in the event that she has other male partners).
Of course, we cannot say for sure that this explanation is correct. Also, we should recognize the limitations of this study, not the least of which is that only a very small number of men participated. Likewise, because the two target women compared were different in so many ways, it is unclear whether a large contrast effect is the driving force between the differences or if we might see the same pattern of effects with any novel female.
More research is certainly needed, but at the very least this study lends support to the idea that the Coolidge Effect is indeed a part of human mating psychology and suggests that maybe, just maybe, there’s an evolutionary reason for it.
Justin Lehmiller, PhD is a sex educator and researcher at Ball State University and author of the blog Sex and Psychology. Follow him on Twitter @JustinLehmiller.
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