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Like all beings, humans have sex to bear offspring. However, unlike most animals that need to go into heat to have sex, we find it enjoyable and interesting all year-round. This behaviour is no coincidence, according to a new paper that suggests being interested in sex all the time acts like a ‘glue’, creating pair bonds between partners. Presumably, this behaviour has been fostered by evolution. The same study also identified a link between the various types of oral contraceptives and how often couples have sex.

Why sex is important

Researchers from the University of New Mexico previously used a dataset comprised of 50 women and their partners. The pairs had to answer a series of questions about their relationships, menstrual cycles, and how often they had sex. No woman involved in this study was using hormonal contraceptives a.k.a. ‘the pill’.

Generally, women were interested in having sex more in the extended sexual phase, when they were not ovulating and the dominant hormone was progesterone. The findings held strong only when the women were invested in the relationship.

A team from the Norwegian University of Science and Technology (NTNU) repeated the experiments performed by their American colleagues, only this time the female participants were all using oral contraceptives. All women used a type of contraceptive that contained hormones that induced a natural cycle.

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The findings were remarkably similar to the original study, showing that how often women are interested in sex is linked to how committed they feel towards their partner.

“The function of sex in humans outside ovulation is an evolutionary mystery. But we believe that it has to do with binding the parties in the relationship together,” says Leif Edward Ottesen Kennair, a professor of psychology at NTNU.

This association was true when the contraceptives had high levels of synthetic hormones that mimic the effects of the natural hormone progesterone, but lowers the levels of estrogen.

“We’re talking about intercourse here, not other types of sex like oral sex, masturbation and such. This strengthens the idea that sex outside the ovulation phase has a function besides just pleasure,” says study co-author Trond Viggo Grøntvedt.

How contraceptives affects sex drive

The findings were not the same, however, when other types of hormonal contraceptives were involved.

All such contraceptives are based on two types of hormones: estrogen and progesterone.

Estrogen naturally peaks around ovulation when women can conceive. Progesterone naturally peaks during the extended sexual phase, a time when offspring can not be conceived. Each type of contraceptive varies the levels of each hormone. So, on one side we have contraceptives that follow ovulation characteristics and simulate a more natural cycle, and, on the other side, we have those that mimic hormones that peak when women can’t conceive.

The researchers found women who use contraceptives with more estrogen were most sexually active when they were in a less committed relationship. Women who used contraceptives based on progesterone were most sexually active when they were faithful and loyal to their partners. It follows that the latter variety of contraceptives induce a ‘nesting’ behaviour.

“Before we did this study, we didn’t know how much difference there was between the two types of hormonal contraceptives,” says Grøntvedt.