What twins experience in the womb could explain why some people are straight and some are gay.

Researchers are trying to discover when sexual orientation first becomes apparent in humans and what might cause it – and they’re looking to twins for the answers.

A controversial study, published in the journal Development Psychology and reported by The Sunday Times, looked at 56 twin pairs with “discordant sexual orientations”.

The participants were asked to provide childhood photographs, so the researchers – Gerulf Rieger and Tuesday Watts from the University of Essex – could examine any differences in expression at various ages.

Sarah Nunn and Rosie Ablewhite, 29, were involved in the study. Their photographs showed great dissimilarities. Where Sarah preferred dresses from a young age, Rosie was commonly seen in dungarees. Where Sarah played with barbies, Rosie liked batman. When the two of them dressed up as characters from The Flintstones, Sarah dressed as Wilma and Rosie as Fred.

“Any boyfriend instantly felt more at home with Rosie,” Sarah told The Sunday Times.

“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 when they tried to get romantic with Rosie she’d say, ‘That’s not me’. Then they came back.”

The researchers found that sexuality differences, or 'divergence', in terms of clothing and self-expression could be seen in the photographs from about the age of six in girls and eight in boys.

"What we can do is rule out a few things now. A lot of people jump to the conclusion it must be genetics," Dr Rieger told The Sunday Times. While previous research has shown genetics has something to do with sexuality, it's not the sole determining factor.

"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."

He doesn't believe it's parenting - because the twins involved in the study were raised by the same parents - but that it might be related to what happens in the womb.

"Prenatal hormones are the number one candidate," Dr Rieger said.

"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."

LISTEN: Anne Stephens had two sets of twins in two years, and shares the greatest stories from the whole shebang. Post continues after audio.