It's in their genes: Tomboys more likely to turn into lesbians because of their genetic make-up.

Genetics, along with hormones and upbringing, may play a part in sexuality, according to a study. Researchers claim it is known that there are well-established traits which differ between girls and boys, with, for example, boys more likely to engage in 'rough and tumble' play.

Children who later become gay or lesbian are more likely to differ from these expected traits, or gender conformity, said a team from Queen Mary University in London. Between 50 to 80 per cent of gender non-conforming boys become gay, and around a third of non-conforming girls become lesbian.

Now, doctors Andrea Burri and Qazi Rahman believe that in girls, genes may be partially responsible for gender non-conformity and, by association, sexual orientation. They followed a group of 4000 female twins, asking them about their sexual attractions and gender non-conformity.

They discovered genetic influences on sexual orientation (25 per cent) and childhood gender nonconformity (31 per cent), reports journal PLoS One. Dr Rahman said: ‘We found that there is a connection between these mental traits and how sexual orientation develops.

‘One idea is that there is an association between these psychological traits and sexual orientation because they all develop under common biological drivers; like the development of brain regions under the influence of genes and sex hormones.

‘We think environmental factors and genetics drive other mechanisms, like exposure to sex hormones in the womb, to shape differences in gender nonconformity and sexuality simultaneously.’

Dr Rahman said the results showed non-conforming behaviour came 'from within', and proved that mental health issues such as depression which have been associated with such behaviour was due to negative reactions from others.

He said: ‘We know that gay people who are strongly gender nonconforming report more anxiety and depression symptoms.

‘Poor mental health in gay populations is partly due to societal stigma and victimisation.

'Our results suggest that being gender nonconforming and lesbian comes from 'within'; there is little you can do about it.

‘So gender nonconformity does not cause mental health problems, but it may trigger negative reactions from other people (like parents and peers) leading to mental health problems.’

He added: ‘Stereotypes like 'sissy' or 'mannish' have not been helpful in promoting respect for gay people, and those who don't match those stereotypes may find it hard to accept they are gay or lesbian.’