There may be an evolutionary reason why some women aren't in the mood.
Forget the headache, the stress or the children - the biggest passion-killer for women, according to new research, is feeling safe and secure once they've got their man. And when they do, their sex drive drops.
After about four years of a relationship, only about one in two women still wants sex regularly. By contrast, men's sex drive remains pretty constant - years into a relationship.
The reason, according to researchers at the Hamburg-Eppendorf University in Germany, is the evolution of the species. Men need to maintain a high sex drive to prevent being usurped by rivals. Women are more interested in establishing a long-term bond to ensure a stable and secure environment for their offspring.
The results, published in Human Nature, challenge modern theories about men and women having equal sex drives.
The researchers, led by psychologist Dr Dietrich Klusmann, questioned more than 500 people aged between 30 and 45 about their libido and how it changed during their relationships.
They were asked how often they wanted sex with their partner and who took the initiative. Nearly two-thirds of 30-year-old women said they wanted sex often at the beginning of their relationship. But after four years, the number dropped to one in two. And after 20 years, it was down to one in five.
By contrast, women's desire for tenderness remained steady, with 90 per cent saying they wanted tenderness from their partner, regardless of the length of their relationship.
The opposite was true of men. Only one-quarter of 30-year-old men still sought tenderness from their partner after 10 years. However, seven in 10 still wanted
sex often, even after 16 years.
The researchers found that women were more likely to maintain their sex drive if their husbands or boyfriends were university educated - something Klusmann attributes to making them a more desirable mate and a target of predatory women.