Sex plays a central role in reproduction, and it can be pleasurable, but new findings suggest that it may serve an additional purpose: Bonding partners together.

A study of newlywed couples, published in Psychological Science, a journal of the Association for Psychological Science, indicates that partners experience a sexual ‘afterglow’ that lasts for up to two days, and this afterglow is linked with relationship quality over the long term.

“Our research shows that sexual satisfaction remains elevated 48 hours after sex,” says psychological scientist Andrea Meltzer of Florida State University, lead author on the study. “And people with a stronger sexual afterglow — that is, people who report a higher level of sexual satisfaction 48 hours after sex — report higher levels of relationship satisfaction several months later.”

Researchers had theorised that sex plays a crucial role in pair bonding, but most adults report having sex with their partners every few days, not every day. Meltzer and colleagues hypothesised that sex might provide a short-term boost to sexual satisfaction, sustaining the pair bond in-between sexual experiences and enhancing partners’ relationship satisfaction over the long term.

To test their hypothesis, the researchers examined data from two independent, longitudinal studies — one with 96 newlywed couples and another with 118 newlywed couples. All of the couples had completed at least three consecutive days of a 14-day daily diary as part of a larger study.

Every night, before going to bed, the newlyweds were asked to report independently whether they had sex with their partner that day. Regardless of the answer, they were also asked to rate how satisfied they were with their sex life that day and how satisfied they were with their partner, their relationship, and their marriage that day (on a seven-point scale, where 1 = not at all, 7 = extremely).

The partners also completed three measures of marriage quality at the beginning of the study and again at a follow-up session about four to six months later.

On average, participants reported having sex on four of the 14 days of the study, though answers varied considerably across participants.

Importantly, sex on a given day was linked with lingering sexual satisfaction over time. Having sex on a given day was linked with sexual satisfaction that same day, which was linked with sexual satisfaction the next day and even two days later. In other words, participants continued to report elevated sexual satisfaction 48 hours after a single act of sex. Importantly, this association did not differ according to participants’ gender or age, and it held even after sexual frequency, personality traits, length of relationship, and other factors were taken into account.

Overall, participants’ marital satisfaction declined between the beginning of the study and the follow-up session four to six months later. But participants who reported relatively high levels of sexual afterglow seemed to fare better relative to their peers, reporting higher initial marital satisfaction and less steep declines in satisfaction across the first four to six months of marriage.

The same pattern of effects emerged in the two independent studies, providing robust evidence for sexual afterglow, Meltzer and colleagues note. Together the findings suggest that sex is linked with relationship quality over time through the lingering effects of sexual satisfaction.

“This research is important because it joins other research suggesting that sex functions to keep couples pair bonded,” Meltzer concludes.