Would you leave your hubby alone in a room with a fertile young woman whose ‘ready-to-mate’ hormones were inviting a roving eye? (Hint: She’s less of a threat than you think.)  

In this study, a group of men—some single and some in committed relationships—interacted with a woman at various points in her menstrual cycle, and then rated her attractiveness. The woman kept her appearance simple (no makeup, ponytail, jeans and a t-shirt), standardized her behavior (neutral responses, no flirtation) and even used unscented beauty products. That way, any changes in her perceived attractiveness would be due to fertility-related cues, like her scent.

Lo and behold, the woman’s fertility provoked opposite reactions depending on a man’s relationship status.

Single men found the woman more attractive when she was highly fertile (the typical response). But committed men bucked the trend: They found her less attractive when she was at her most fertile.

Why? Men in committed relationships may downplay the attractiveness of a potential new mate in order to preserve their relationships—an unconscious effort to stay faithful. What a refreshing idea!