A woman's sex drive begins to plummet once she is in a secure relationship, according to research.
Researchers from Germany found that four years into a relationship, less than half of 30-year-old women wanted regular sex.
Conversely, the team found a man's libido remained the same regardless of how long he had been in a relationship.
Writing in the journal Human Nature, the scientists said the differences resulted from how humans had evolved.
The researchers from Hamburg-Eppendorf University Hospital interviewed 530 men and women about their relationships.
They found 60% of 30-year-old women wanted sex "often" at the beginning of a relationship, but within four years of the relationship this figure fell to under 50%, and after 20 years it dropped to about 20%.
In contrast, they found the proportion of men wanting regular sex remained at between 60-80%, regardless of how long they had been in a relationship.
Tenderness
The study also revealed tenderness was important for women in a relationship.
About 90% of women wanted tenderness, regardless of how long they had been in a relationship, but only 25% of men who had been in a relationship for 10 years said they were still seeking tenderness from their partner.
Dr Dietrich Klusmann, lead author of the study and a psychologist from Hamburg-Eppendorf University Hospital, believed the differences were down to human evolution.
He said: "For men, a good reason their sexual motivation to remain constant would be to guard against being cuckolded by another male."
But women, he said, have evolved to have a high sex drive when they are initially in a relationship in order to form a "pair bond" with their partner.
But, once this bond is sealed a woman's sexual appetite declines, he added.
He said animal behaviour studies suggest this could be because females may be diverting their sexual interest towards other men, in order to secure the best combinations of genetic material for their offspring.
Or, he said, this could be because limiting sex may boost their partner's interest in it.
Professor George Fieldman, an evolutionary psychologist from Buckinghamshire Chilterns University College, said: "These findings seem to fit in with anecdotal studies and his explanations seem plausible.
"The rational for why a woman's sex drive declines may be down to supply and demand. If something is in infinite supply, the perceived value would drop."
