The study, led by Dr Gerulf Rieger from the Department of Psychology at the University of Essex, involved 345 women whose responses to being shown videos of naked men and women were analysed.
The results, which were based on elements such as whether their pupils dilated in response to sexual stimuli, showed that 82% of the women tested were aroused by both sexes.
Meanwhile of the women who identified as straight, 74% were strongly sexually aroused by videos of both attractive men and attractive women.
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.”