Sets of identical twins where one is straight and the other is gay are being studied to work out how sexuality is formed.

Sarah Nunn and Rosie Ablewhite, both 29, were interviewed in the Times this weekend.

Sarah is straight and Rosie is a lesbian, but they are genetically identical and were raised together in the same household.

Scientists are using sets of twins like Sarah and Rosie to try to unravel the nature vs nurture argument when it comes to human sexuality.

“Any boyfriend instantly felt more at home with Rosie,” recalled Sarah, speaking to the Times.

Adding: “She liked football, talked about boy things, played video games. They’d be like, ‘Sarah, you’re really boring. I’m going to go and play with Rosie.’ I’d get jealous that they liked her better.”

But she went on: “When they tried to get romantic with Rosie she’d say, ‘That’s not me.’ Then they came back.”

The scientists at the University of Essex, Gerulf Rieger and Tuesday Watts, used photographs from childhood for the controversial test.

They then asked strangers to see if they could tell whether the twins had discordant sexual orientations.

Some have criticised the work, suggesting that it reinforces gender stereotypes.

But it suggests that markers of sexual orientation manifest before puberty.

“What we can do is rule out a few things now. A lot of people jump to the conclusion it must be genetics,” says Dr Rieger.

“This shows there is something early on, in the early environment, that has nothing to do with genes but can still have a tremendous effect on sexual orientation.”

Dr Rieger adds that sexual orientation could be determined before birth, explaining why identical twins can have different sexual orientations.

“Prenatal hormones are the number one candidate,” he says.

“Our theory is that even though twins are identical, what happens in the womb can be quite different. They can have different nutrition, different levels of hormones.”

Another set of twins, Matilda and Lily, 30, were also involved in the study, and said they anticipate that the study could answer one of their biggest unanswered questions.

“My mum and dad were fascinated. How can it be that one egg split and in such a large factor of our life we were programmed so different?” says Matilda.