We all can be forgiven if we sometimes forget that sex – despite what the poets, songwriters and HBO all have to say about it – is ultimately about reproduction. We are built to want sex because it is a zoological imperative – all species must reproduce to stay in the game.

But what about that warm, post-coital feeling we often get after sex, that can linger for days? That's not biology, that's... romance, isn't it?

Apparently not.

Scientists at Florida State University have concluded that our after-sex afterglow is a design of evolution that provides a short-term boost to sexual satisfaction, sustaining the pair bond in between sexual experiences and enhancing partners’ relationship satisfaction over the long term.

“Our research shows that sexual satisfaction remains elevated 48 hours after sex,” says psychological scientist Andrea Meltzer, 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.”

The researchers drew upon two separate studies, one with data from 96 newlywed couples, the other featuring 118 newlywed couple. In each study, the newlyweds were asked to report independently whether they had sex with their partner that day. Regardless of their answers, 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. The questionnaire was completed at the start of the study and later at a follow-up session 4 to 6 months later.

The data showed how having sex on any given day was linked with satisfaction that same day and with sexual satisfaction up to 48 hours later. This connection did not vary with the participants age or gender, and it was consistent regardless of sexual frequency, the length of the couple's relationship and various personality traits.

In short, the sexual satisfaction that carries across as an afterglow affects all of us.

Where there was variation, was in the intensity of the afterglow. The higher the level of sexual afterglow, the higher the initial marital satisfaction and less steep declines in satisfaction reported across the first 4 to 6 months of marriage.

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

The research was published in Psychological Science, a journal of the Association for Psychological Science