09.24.131:17 PM ET

Women ‘Sniff Out’ Competition

Study: Smelling a woman during her period brings out aggression.

Watch out for cat fights while on your period. According to a new study in Evolution and Human Behavior, when women were exposed to T-shirts worn by other females during their high-fertility days the women had higher levels of testosterone. When the women smelled T-shirts worn by low-fertility females, their testosterone levels dropped significantly. Since testosterone is linked to aggression, this could lead to women acting more competitive during that time, says the study’s lead author, Jon Maner. “Humans are influenced much more strongly by ovulatory cues than we tend to think,” Maner says. In addition, one of Maner’s earlier studies saw that men were more risk-prone and flirty when in the presence of a research assistant during her menstrual cycle.