The researchers hypothesize that the facial hair "may signal men's age and masculine social dominance."
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As such, it could indicate "formidability among males and the potential to provide direct benefits, such as enhanced fertility and survival, to females."
For the experiment, 8,520 women were asked to rate a variety of images of male faces for physical attractiveness through the perspective of a "short-term liaison or a long-term relationship."
As a news release about the research states, "Stubble was judged as most attractive overall and received higher ratings for short-term relationships than full beards, which were more attractive for long-term relationships. Extremely masculine and extremely feminine-looking males were least attractive, irrespective of relationship context."