According the journal Human Nature a woman's sex drive begins to plummet once she is in a secure relationship. 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%.

Conversely, they found the proportion of men wanting regular sex remained at between 60-80%, regardless of how long they had been in a relationship.

The scientists said the differences resulted from how humans had evolved. For men, a good reason their sexual motivation to remain constant would be to guard against another male taking their place. However, women 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.