In a study done by researchers at Hamburg-Eppendorf University in Germany, researchers found that a women's libido starts to wane as soon as she feels secure in a relationship.
Researchers found that 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 that men's sexual desire remains just as strong regardless of how long they have been with their partner.
The study by researchers is reported in the journal Human Nature. The study was done among 530 men and women about their relationships. Dr Dietrich Klusmann, lead author of the study and a psychologist from Hamburg-Eppendorf University, believed the differences were down to human evolution.
He said, "For men, a good reason for their sexual motivation to remain constant would be to guard against being cuckolded by another male." On the other hand, women 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.
The study also showed that women's desire for tenderness remained constant over the years whereas only 25% of men who had been in a relationship for 10 years said they were still seeking tenderness from their partner. The study also showed that living apart helped to keep a women's sex drive higher to some extent.