The finding helps explain why homosexuality doesn’t die out, even though lesbian couples are not able to naturally have children carrying the genes of each partner.

This is because straight tomboys have more lovers than others – and so the ‘tomboy DNA’ stays in circulation, according to the study carried out by a team from King’s College London.

Researchers questioned almost 500 pairs of twins about whether they were attracted to men or women, how many lovers they had had and about their levels of ‘childhood gender non-conformity’ – if they were a tomboy and whether they preferred to play with boys rather than other girls.