Most women are never straight - they are either gay or bisexual, a study suggests.
Although lesbians are much more attracted to females, most women who say they are straight were aroused by videos of both naked men and naked women.
The study, led by Doctor Gerulf Rieger from the Department of Psychology at the University of Essex, showed a series of videos of naked men and women to 235 women and recorded their responses.
It found 82 per cent of the women tested were aroused by both sexes, based on indicators such as whether their pupils dilated in response to sexual stimuli.
Of the women who identified as straight, 74 per cent were strongly sexual aroused by videos of both attractive men and attractive women.
In contrast, lesbians showed much stronger sexual responses to women than to men.
"Even though the majority of women identify as straight, our research clearly demonstrates that when it comes to what turns them on, they are either bisexual or gay, but never straight," Dr Riegler said.
He also said the study showed lesbians who may dress in a more masculine manner may not have more masculine behaviours.
"Although some lesbians were more masculine in their sexual arousal, and others were more masculine in their behaviours, there was no indication that these were the same women.
"This shows us that how women appear in public does not mean that we know anything about their sexual role preferences."
A YouGov survey previously found almost half of young people in the UK do not identify as 100 per cent straight.
It also found 41 per cent of people identify themselves as somewhere between homosexual and exclusively heterosexual - suggesting people are increasingly seeing sexuality in a less polarised manner.
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