Women who think they are heterosexual are actually bisexual or gay and "never straight", researchers have suggested.
The study from the University of Essex explored what really turns women on.
According to the researchers, it is the first time that the differences between straight and gay women have been "clearly defined".
The study analysed the physiological sexual responses of 345 women, using eye tracking devices.
Earlier this year, researchers from the University of Essex found a 100% link between pupil dilation and sexual arousal. They concluded that a person's eyes become more dilated when they are aroused.
In the latest study, the scientists found that women who identified as heterosexual were strongly sexually aroused to videos of both attractive men and attractive women, which was identified by their pupils dilating.
In contrast to the response from heterosexual women, lesbians showed much stronger sexual responses to women over men.
Researchers say lesbians are more like men when it comes to being aroused than straight women, as men "usually show distinct sexual responses to their preferred sex".
The study, which was published in the Journal of Personality and Social Psychology, also set out to test the theory that because lesbians can be more masculine in many of their non-sexual behaviours (for example, the way they dress), they are also more masculine in their sexual responses.
However the research, led by Dr Gerulf Rieger, did not find any evidence of this.
Dr Rieger, who is from the Department of Psychology, explained: "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. Men are simple, but women’s sexual responses remain a mystery."
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