The new research by the University of California has not yet been published but is being presented at the annual meeting of the American Society of Human Genetics in Baltimore.

Scientists studied 37 sets of identical male twins, who were born with the same genetic blueprint, to tease out which genes were associated with homosexuality. In each pair, one of the twins was gay.

Only 20 percent of identical twins are both gay leading researchers to believe that there must be causes which are not inherited.

They found that it was possible to tell whether a man was gay or straight by monitoring tiny changes in how his DNA functions after birth – a field known as epigenetics. Where DNA works as an overall instruction manual, epigenetics act as another layer of information highlighting which parts of the text are important and which can be ignored.