Scientists searching for the root of human sexuality studied identical twins, one of them straight and the other a lesbian, who grew up in the same environment.
Sarah Nunn and Rosie Ablewhite, 29, present a mystery for researchers looking to identify genetic and environmental interactions that form sexuality because Sarah is attracted to men while Rosie is attracted to women.
The origins of their differing sexual identities were studied in an effort to find out when and how sexuality develops in childhood.
Sarah, right, and Rosie, left, were at the centre of a study by scientists trying to work out the root of human sexuality
Sarah remembers how Rosie's tomboy tendencies provided an insight when they were growing up, telling The Times her boyfriends 'instantly felt more at home' with her sister.
'She liked football, talked about boy things, played video games,' she said.
'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 Sarah soon realised that her sister just wasn't as interested as her in the company of boys.
'When they tried to get romantic with Rosie she’d say, "That’s not me." Then they came back,' she explained.
Scientists are studying genetically identical twins who diverge in sexuality in order to establish what causes sexual preferences
Now they and 55 other twin pairs are at the centre of a study by University of Essex researchers.
In the past, scientists have searched for signs of how sexuality manifests before puberty, such as gender-atypical mannerisms of behaviour.
But it is difficult to determine whether reported behaviourial patterns have been remembered accurately.
Gerulf Rieger and his colleague Tuesday Watts circumvented this problem by using photographs in their study for the Developmental Psychology journal.
They asked Sarah, Rosie and other twins with 'discordant sexual orientations' to send them childhood snaps, before showing them to people who did not know the purpose of their experiment.
The identical twins were brought up in the same household, ruling out parenting and genetics as the causes of their difference
The idea was to see if people could spot how and when the twins diverged just from looking at their clothing and play.
Their questions are controversial, as establishing links between sexuality and other aspects of gender could be seen as reinforcing stereotypes about male and female behaviour some believe are harmful.
But these stereotypes are difficult to avoid in the pictures submitted by Sarah and Rosie.
As toddlers, Sarah is seen wearing a dress and playing with a Barbie, whereas Rosie dons a Batman suit and plays with Aladdin.
Later in childhood, Sarah dresses up as The Flintstones character Wilma, but Rosie opts for Fred. Throughout her early years Rosie's favoured attire is dungarees.
And 20 years on the twins are puzzled as to how it took so long to realise they were different.
It seemed impossible to them that they could diverge on something so fundamental while being identical twins.
Rosie remembers questioning why she didn't feel the same passion for boys as her sister.