My HormonologyNext time you find yourself attracted to a guy with a scar on his face, think about where you are in your menstrual cycle. Chances are, you’re in Week 2, which starts 8 days from the onset of your period. And you’re likely thinking of the guy as more suitable for a short-term fling than a long-term romance.

Why? According to a study in the journal Personality and Individual Differences, facial scars on men can be an indication of higher testosterone levels because scarred-up guys tend to be braver and more rough-and-tumble. (Just think of some of the manly men who sport them, like Harrison Ford, David Beckham and Joaquin Phoenix.) And research shows that during Week 2, which is when fertility is high, heterosexual women are more interested in brief affairs with higher-testosteroned males because they tend to be healthier, more suitable genetic fits for your potential offspring, however, also they tend to be less committed, conscientious partners.