It is not quite clear why human males continue to have facial hair. There are other ways to keep warm, and we have lost our hair over most of the rest of our bodies (or they are at best residual). But facial hair serves another important purpose - determining male attractiveness to females.

Different types of facial hair trigger different reactions among women, who may consciously or unconsciously make decisions on mate selection, parenting ability, health, and masculinity based on how thick a beard is. However, these exact decisions often have been contradictory in studies on attractiveness. In a recent study published in Evolution and Human Behaviour, Barnaby Dixson and Robert Brooks from the University of New South Wales measured women's and men's impressions of facial hair, and attempted to make sense of previous, confusing results. They presented photographs of 10 smiling men in various stages of beard growth, to 177 heterosexual men and 351 women, 80% of whom were of European descent. 

Women found heavy stubble to be the most attractive, while heavy beards, light stubble and no beard were all equally less attractive. Men, however, found full beards and heavy beards as most attractive. Light stubble was rated least attractive by both genders. Regarding the beard-wearers' parenting ability and healthiness, both men and women rated full beards as the best choice. Masculinity assessments correlated with amount of facial hair (the more hair, the more men were perceived as masculine). And while masculinity ratings of heavy beard-wearers were particularly high among women in the fertile phase of their menstrual cycles, overall attractiveness did not depend on the women's fertility. "An intermediate level of beardedness is most attractive, while full-bearded men may be perceived as better fathers who could protect and invest in offspring," the authors wrote.

The study suggests that facial hair plays an evolutionary role in selecting mates, and that decisions based on these physical characteristics is ongoing. The researchers also noted significant differences in how men and women perceive facial hair. "Facial hair correlates not only with maturity and masculinity, but also with dominance and aggression," Dixson and Brooks wrote. "Men, judging other men, might be sensitive to the overall level of masculine threat and aggression signaled through full beards. Women, by contrast, may balance [...] a competitive masculine partner against the costs of mating with a too-masculine partner." 

Fertility didn't relate to attractiveness ratings of beards, which shows that there is a separation of preferences from reproductive status; this is unusual because most sexual traits that are based on hormones are closely tied to reproductive status. The researchers state that more work is needed to understand the relationships between female fertility status and impressions of sexual characteristics.