Evidence for the stress-linked immunocompetence handicap hypothesis in humans.

Secondary sexual traits that develop under the action of testosterone, such as masculine human male facial characteristics, have been proposed to signal the strength of the immune system due to the sex hormone's immunosuppressive action. Recent work has suggested that glucocorticoid stress hormones may also influence expression of such sexual signals due to their effects on immune function. Precise roles, however, remain unclear. Here we show positive relationships between testosterone, facial attractiveness and immune function (antibody response to a hepatitis B vaccine) in human males, and present some preliminary evidence that these relationships are moderated by naturally co-occurring cortisol (a glucocorticoid stress hormone involved in the fight-or-flight response). We conclude that our results provide support for a role of glucocorticoids in hormonally mediated sexual selection.