Of course, sex is about reproduction, getting your genes into the next generation. But sometime in our deep primordial past, our pre-human ancestors decoupled sex from reproduction. Very few human acts of sex are performed for the intended purpose of making a baby. In fact, we go to extensive lengths to avoid pregnancy, and we’ve devised various ingenious methods of contraception throughout the ages.

Evolutionary scientists generally believe that non-reproductive sex evolved in humans for the purpose of parental pair-bonding. In the animal world, the deadbeat dad is the norm. It’s wham, bam, thank you ma’am—and good luck with the kids!

But for the most part, human dads stick around, and not just out of guilt or a sense of duty. They love their children and want to play an active role in raising them. Still, it’s not clear why they do this.

Since the game of life is all about getting your genes into the next generation, it makes sense that the males would want to skedaddle as soon as they’ve contributed their few drops of genetic material to the baby-making process. Mothers are stuck with their young ones, but if daddy sticks around, he foregoes the opportunity to contribute his genetic material to another female’s offspring.

However, parenting for Homo sapiens is far more demanding than it is for other species. Human babies are still quite underdeveloped at birth, and they take longer to reach sexual maturity than any other species. Especially in prehistoric times, a single mom had little chance of successfully raising a child on her own. The solution was to find a way to keep her child’s father around.

And so the human mating contract was forged. It goes like this: The woman consents to keep having sex with the man, and in return the man helps raise the kid. Long before the institution of marriage was invented, pair-bonding for the purpose of raising children was the norm among humans.

As fun as it is, you just can’t have sex all the time. It’s physically demanding, and sooner or later you’re exhausted. Muscles get sore, tender tissues get chafed. Besides, you’ve got to go out and find food every day. And don’t forget the offspring begotten from all these sexual escapades. We all know how much kids put a damper on our sex life.

So there’s got to be more than just sex to keep a couple bonded together. At least, so argues Florida State University psychologist Andrea Meltzer and her colleagues in a recent article in the journal Psychological Science. Sex feels great, they maintain, but the lingering afterglow of sex feels pretty good too. And that’s what really keeps the couple together, since after all the sex is great in a one-night stand as well.

But how long does the afterglow of sex last? This is precisely what Meltzer and her colleagues set out to measure. The researchers recruited newly-wed couples and asked them to fill out a general survey on relationship satisfaction. Then, for the next 14 days, each partner in the couple responded separately to a brief questionnaire that asked, among other things, whether they’d had sex with their partner that day, and how satisfied they currently were with their relationship and sex life.

(On a side note, one interesting finding was that the partners didn’t always agree on having had sex that day. There are multiple explanations for this anomaly, such as different opinions on what acts count as sex. The researchers counted a sexual act as having occurred only if both partners said it did.)

After four to six months had passed, each partner in the couple again completed a general survey of relationship satisfaction. When the researchers compared responses on the initial and follow-up surveys, they found the honeymoon doesn’t last very long. Couples were generally less satisfied with their relationships after they’d been married for several months.

Everybody wants to know how their marriage stacks up to those of other people, so here are some benchmark statistics from this study:

  • On average, couples had sex on 4 out of the 14 days of the survey, or twice a week. Whether you think this frequency high or low, keep in mind that these were couples who’d been married less than 4 months.
  • The frequency ranged widely, with some newly-wed couples reporting no sexual activity at all during the two-week period and others claiming they had sex every day.
  • Overall, the participants were satisfied with their sex life, but most just moderately so.

And now for the answer to the key question: How long does the afterglow of sex last? Not surprisingly, participants reported a high level of sexual satisfaction on days they’d had sex with their partner. They also reported a high level the day after. But by the third day, sexual satisfaction had dropped to baseline levels. So there you go: The afterglow of sex lasts about 48 hours.

Since participants reported sexual satisfaction on a seven-point scale, the researchers could also measure the strength of afterglow. While participants tended to report high levels of relationship satisfaction on days they also reported high sexual satisfaction, an analysis of the data showed that it was daily sexual—and not relationship—satisfaction that predicted marital happiness both at the beginning of the study and during the follow-up some months later.

In other words, marriages are happiest when the sex the couples engage in led to a strong, sustained afterglow. These findings suggest that relationship satisfaction depends not only on the quantity of sex but also on the quality of it. When it comes to marital happiness, it seems that frequent sex is not enough—it also has to be memorable.

References

Meltzer, A. L., Makhanova, A., Hicks, L. L., French, J. E., McNulty, J. K., & Bradbury, T. N. (2017). Quantifying the sexual afterglow: The lingering benefits of sex and their implications for pair-bonded relationships. Psychological Science, 28, 578-598.

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