Of all the simple dichotomies we use to define ourselves and the world we inhabit, perhaps none is so basic and sometimes destructive in its effects as the distinction between males and females. But what exactly do we mean by "masculinity" and "femininity"? Even if researchers can agree on the meaning of these terms, how do they determine whether the behavior they observe arises from intrinsic differences between males and females or from the context in which it occurs? What is the balance between biologically and environmentally influenced sex differences? In this volume a group of internationally renowned authorities on sexual differentiation consider these and other key questions from seven distinct but interrelated perspectives: psychobiologic, neuroscientific, evolutionary, behavior genetic, developmental, psychosocial, and cultural. Specific topics range from sex differences in the rodent brain to the psychological constructions of masculinity and femininity. The contributors' findings indicate that the characteristics of each gender are determined by a much more intricate interaction between biology and the environment than has been accepted in the past, and they relate these findings within a framework of critical issues in methodology and definition. Psychologists, neuroscientists, psychiatrists, and social scientists will welcome this fresh and authoritative contribution to the study of sexual differentiation.