Research: A woman's sexuality may depend on her romantic options

“I do not claim that women become lesbians because they are not attractive enough to get men,” McClintock told Live Science in an email. “One could just as easily imply that some women never have the opportunity to partner with women because they are caught up in the pressure of hetero-normativity,” or the pressure to adhere to straight norms.

Rather, the study suggests that women who are rated as less attractive may feel less pressure from straight norms and so are free to explore same-sex attractions.

If sexual orientation is a spectrum, with absolute straight or gay attraction on either end, then more women than men are in the middle of the curve. Evolutionary psychologists have proposed that women may be more heteroflexible because it made good evolutionary sense. Women who were raped or whose partners ran off or died would have better success if they paired up with another woman to raise their children, rather than going it alone.