Hormonal clues to evolutionary puzzle – why do some women have more sex when at their least fertile?

Women in long-term, committed relationships tend to have a higher sex drive if they use hormonal contraceptives - such as the pill or injection - based mainly on progesterone, a new study finds.

People who use contraceptives that are mainly based on oestrogen, on the other hand, had higher sex drives when they were in less committed relationships, according to a paper published in the journal Evolution and Human Behaviour.

The study looked at two groups of women who used hormonal contraceptives – one group of 112 women answered surveys over three months, reporting how many times they had sex and when in their cycles. A second group of 275 women in long-term relationships answered a one-off survey to find out how many times they had had sex in the past week.

Both groups were also asked which hormonal contraception they were using and about the kinds of sex they were having.

The scientists were trying to answer the question of why women have sex outside of oestrus – the time when women are most fertile and likely to conceive – and why some have it more than others.

"Cats and dogs, for example, do not have sex outside of oestrus," study author Leif Edward Ottesen Kennair, a psychologist at the Norwegian University of Science and Technology, told IBTimes UK. "The mystery is what function would this sexuality have in humans?"

A previous study found that women who did not use hormonal contraception had more penetrative sex outside of oestrus when they were in committed relationships. The present study confirmed that women who used hormonal contraceptives also had more sex outside of oestrus if they were in a committed relationship.

Penetrative sex was the only type of sex to increase outside of oestrus in these women, Kennair says. Levels of masturbation and oral sex did not show the same relationship.

As this was not a clinical study, women should not change their hormonal contraception in order to alter their sex drive based on this study, Kennair says. "The findings seem to be robust but at this point it would be beyond the scope of the paper to start giving people advice on what hormones to take."

Kennair says that "the dream would be to follow up with a clinical intervention study", to see whether it could be an option for women to make changes to their sex drive by choosing different hormonal contraceptives. However, his next steps are to replicate the findings and find out more about the relationship between the hormones and factors such as faithfulness and commitment in relationships.