Controversial evolutionary psychologist  Dr. Satoshi Kanazawahas written a new evolutionary psychology paper arguing 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.

This is because it helps in reducing conflict and tension inherent in such marriages while at the same time successfully reproducing with their husbands in heterosexual unions.

Kanazawa also says this would 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.

He seems to have concluded this by having his belief and finding data in the National Longitudinal Study of Adolescent Health (Add Health) to confirm it, which is terrible science but common in psychology. He used his hand-picked statistic to find that women (but not men) who experience increased levels of sexual fluidity have a larger number of children, suggesting to him that female sexual fluidity, if heritable, may be evolutionarily selected. He also determined that women (but not men) who experience marriage or parenthood early in adult life subsequently experience increased levels of sexual fluidity and that sexual fluidity is significantly positively correlated with known markers of unrestricted sexual orientation among women whereas it is significantly negatively correlated with such markers among men.

"The theory suggests that women may not have sexual orientations in the same sense as men do," said Dr. Kanazawa, author of the Biological Reviews article who nonetheless does not know what the word "theory" means. "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."