Could twins hold the answer to the much-debated question of where human sexuality comes from? Scientists think they might provide a clue. A study of identical twin sisters – where one is gay and the other is straight – has been conducted to work out how and when human sexuality is formed.
For a paper published in the journal Developmental Psychology, researchers at the University of Essex studied 56 pairs of identical twins with “discordant sexual orientations” by looking at childhood photos for signs of gender-atypical mannerisms and behaviour, reported The Times. However, some have criticised the study for implying that sexuality is linked to other aspects of gender and therefore reinforces gender stereotypes.
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The study's conclusion? That differences in sexuality were visible from around age six in girls and eight in boys, suggesting markers of sexual orientation show up before puberty.
Identical 29-year-olds Sarah Nunn and Rosie Ablewhite, who were brought up together, were among the twins studied. Their childhood photos show the pair as toddlers with Sarah, who is straight, wearing a dress and playing with a Barbie, while Rosie, dressed as Batman, played with Aladdin. Later on in their childhood, Sarah dressed as Wilma from The Flintstones, while Rosie dressed as Fred.
“Any boyfriend instantly felt more at home with Rosie,” Sarah told The 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 boys "tried to get romantic" with her sister, Rosie would brush them off and they'd "come back" to Sarah, she continued.
Rosie, meanwhile, said she had long questioned the difference between them. “No offence, Sarah was really boy crazy,” she told The Times, adding that while she tried to be like her sister for a while, she soon realised that whenever she had a boyfriend she didn't want to kiss him.
The researchers said their work rules out the idea that sexuality is solely the product of either nature (genes) or nurture (the environment). Identical twins, who share all their genes, are more likely to both be gay or both be straight than non-identical twins, who share half their genes, which suggests genes play a part in sexuality. However, the fact that identical twins, like those studied, can have different sexualities suggests the environment plays a part, too.
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“What we can do is rule out a few things now. A lot of people jump to the conclusion it must be genetics,” said Dr. Gerulf Rieger, one of the researchers. “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.”
Sexual orientation could be determined before birth, he continued, with prenatal hormones being "the number one candidate". “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.”
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