Tempests and tales: challenges to the study of sex differences in the brain

Jordan-Young wrote Brain Storm after she became interested in the causes of variation in human sexual behavior while engaging in a study of human sexuality related to the acquired immunodeficiency syndrome epidemic. During the course of this work, she was struck by the substantial variation that is exhibited by humans in relation to sexual behavior and the difficulty one can encounter in fitting individuals neatly into categories such as man versus woman or homosexual versus heterosexual. In this context, she was fascinated by the claim that there might be a "male or female" or a "gay versus straight" brain. Her interest was piqued by Simon LeVay's 1991 Science paper [7] because it employed a very simple categorization of its subjects into "men, women and gay men." She questioned the value of such a scheme, given the lack of sharp categorical boundaries she had experienced in her own work. When trying to understand the possible causes of correlations between brain structure and human sexual behavior identified by Gorski et al. [8], LeVay [7] and Swaab [9], to name a few, she learned about the organizational and activational hypothesis proposed by Phoenix et al. in 1959 [10]. The now iconic organizational and activational hypothesis codifies the concept that early hormone exposure permanently organizes the neural substrates which will be activated in a sex-specific manner in adulthood by gonadal steroid hormone production. As most readers of this journal will know, whether this hypothesis can be applied to the human brain remains a matter of controversy and a difficult question to address, given the inability to perform the same types of experiments that have clarified the validity of the hypothesis in other species. Jordan-Young reviews, in a thorough and engaging manner, the challenges and pitfalls of trying to study brain sexual differentiation in humans. This is one of the strongest aspects of the book. Human behavior is complex and often does not exhibit marked