Biologists characterize beards as a secondary sexual characteristic because they are unique to one sex, yet do not play a direct role in reproduction. Charles Darwin first suggested possible evolutionary explanation of beards in his work The Descent of Man, which hypothesized that the process of sexual selection may have led to beards. Modern biologists have reaffirmed the role of sexual selection in the evolution of beards, concluding that there is evidence that a majority of females find men with beards more attractive than men without beards.
Evolutionary psychology explanations for the existence of beards include signalling sexual maturity and signalling dominance by increasing perceived size of jaws, and clean-shaved faces are rated less dominant than bearded. Some scholars assert that it is not yet established whether the sexual selection leading to beards is rooted in attractiveness (inter-sexual selection) or dominance (intra-sexual selection). A beard can be explained as an indicator of a male's overall condition. The rate of facial hairiness appears to influence male attractiveness. The presence of a beard makes the male vulnerable in fights, which is costly, so biologists have speculated that there must be other evolutionary benefits that outweigh that drawback. Excess testosterone evidenced by the beard may indicate mild immunosuppression, which may support spermatogenesis.