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."