What makes a man attractive to a woman? This, truly, is a question as old as our time on this planet. Now scientists at the School of Psychology at The University of Queensland in Australia think they may have doped out an answer – for both short- and long-term relationships.

These aren't the first researchers to go exploring down this path. Previous studies have shown that men with more masculine facial features, such as a more pronounced brow bridge or wider jawline, were considered more attractive. Other studies contradicted these findings, indicating that women preferred men with softer facial features, as they perceived them to be more considerate and caring. Still other studies hedged their bets, declaring that women dated cavemen, looking for more masculine men for short-term relationships,  but married the poets, trusting to the sensitive types for the long term.

The team from Queensland believes they have finally settled the science. They photographed 36 men at three stages of facial hair – clean shaven, light stubble (5 days of hair growth), heavy stubble (10 days of hair growth), and a full beard (at least 4 weeks of untrimmed hair growth). They further tweaked each photograph to look either more or less masculine. The researchers next asked 8,520 women to view the normal and Photoshopped photographs and rate how attractive they thought each man was in general. They also had to indicate how attractive they found each man for short- and long-term relationships.

"The 'short-term attractiveness' condition asked participants to rate the men when imagining the type of person who would be attractive in a short-term relationship. This implies that the relationship may not last a long time," the authors noted.

"The 'long-term attractiveness' condition asked participants to imagine they were looking for the type of person who would be attractive in a long-term relationship. Examples of this type of relationship would include someone you may want to move in with, settle down and, at some point, wish to marry," they added.

The results may surprise you. Men with clean shaven, masculine faces were viewed as least attractive. The next least attractive were feminine faces. Most attractive? Unmanipulated faces with light and heavy stubble, beating out clean-shaven and full-bearded men. 

As far as relationships go, the women participating in the study rated full-bearded men as most attractive for long-term relationships, while light and heavy stubble were considered most attractive for short-term relationships.

The researchers concluded: "Our findings suggest that beardedness may be attractive when judging long-term relationships as a signal of intrasexual formidability and the potential to provide direct benefits to females.

“More generally, our results hint at a divergence of signaling function, which may result in a subtle trade-off in women's preferences, for two highly sexually dimorphic androgen-dependent facial traits."

The study was published in the Journal of Evolutionary Biology.