Although the physiology and psychology of women and men are almost identical, the small differences, attributable mainly to sex hormones, influence mood and behavior and lead to differential responses on the part of others. Most cultures, throughout history, have institutionalized these differences and encouraged the evolution of two separately elaborated social roles, one for women and one for men. This has resulted in many inequities, notably unequal opportunities in the workplace, which have had further effects on the consequences of sex on the individual. At this stage of cultural evolution, it is almost impossible to disentangle the role of hormonal difference from the many layers of social attributions that have magnified and complicated this difference. From a health viewpoint, however, it is important, when it comes to sex, to try to sweep away the elaborate and enticing cobwebs of role expectations and biased perceptions and carefully to examine what makes men and women hormonally different and how this difference influences emotion, behavior, prevention of illness, psychiatric morbidity, optimal treatment strategies, and, ultimately, outcome of illness. It is also tempting to think that, by comparing men and women who become depressed, who develop schizophrenia, or who become demented, we will be able to probe deeper into the etiologies of these, as of now, mysterious disorders.