Most women are either bisexual or gay but "never straight", research suggests.
Women who say they are straight are in fact aroused by naked men and naked women, whereas lesbians are much more attracted to the female form.
The study, led by Dr Gerulf Rieger from the Department of Psychology at the University of Essex, involved 345 women.
They were shown videos and had their responses recorded, including whether their pupils dilated in response to sexual stimuli. Straight women were strongly sexually aroused by videos of attractive men and attractive women, even though they said they were only interested in men.
This was in contrast to lesbians, who showed much stronger sexual responses to women than to men.
The researchers said lesbians were the most like men in their responses because it is usually men who show distinct sexual responses to their favourite sex.
Dr Rieger said: "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."