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A new evolutionary theory argues that women may have been evolutionarily designed to be sexually fluid—changing their sexual desires and identities from lesbian, to bisexual, to heterosexual and back again—in order to allow them to have sex with their co-wives in polygynous marriages, therefore reducing conflict and tension inherent in such marriages while at the same time successfully reproducing with their husbands in heterosexual unions.

The theory may also help explain a number of puzzles in human sex research, including differences in female and male homosexuality, male arousal to lesbian sex, and menstrual synchrony.

"The theory suggests that women may not have sexual orientations in the same sense as men do," said Dr. Satoshi Kanazawa, author of the Biological Reviews article. "Rather than being straight or gay, to whom women are sexually attracted may depend largely on the particular partner, their reproductive status, and other circumstances."