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.