Scientists at Indiana University found that women’s brains respond to male faces differently based on where the woman is in her menstrual cycle. 

Twelve heterosexual women ages 23 to 28 who were not using hormonal contraceptives nor in committed relationships were shown 256 photos of male faces that varied in facial masculinity and sexual risk.

Researchers measured the women’s brain activity with an fMRI scanner while they were viewing the photos--once when they were ovulating and again at another point in their cycle.