Brain Gender is a fascinating book, clearly written and well organized. The author, Melissa Hines, professor of psychology and current director of the Neuroendocrinology Research Unit at London's City University, traces her interest in sex differences and their origins to her freshman year in college, when she and other women, the first to be admitted to Princeton, were assigned to "two-man rooms" and were routinely addressed as "Mr." by unthinking preceptors. Hines trained at UCLA and the University of Wisconsin in personality theory, developmental psychology, and neuroendocrinology. Brain Gender reflects not only the author's mastery of developmental biology—particularly in relation to hormonal influences on brain development and plasticity, developmental anatomy, and sex-related behaviors in multiple species—but also her understanding of the social and cultural implications of sex and gender differences for the human species.

Hines reviews carefully and lucidly the research to date on sex differences in humans, starting with genital ambiguity and its relationship to genetic and hormonal abnormalities. She establishes that gonadal hormones play a major role in tissue organization and the structural development of genitalia in human sexual differentiation; she then examines the influence of gonadal hormones on neural and behavioral development in other mammals. Hines supplies an admirable summary of intriguing animal research illustrating both the "organizing" and the "activating" nature of hormones. For example, female rats located "downstream" in terms of uterine blood flow from male littermates show as adults more male-typical behaviors, such as mounting, than females located "upstream."

Hines also notes the importance of feedback loops and regulatory mechanisms, some of which are environmentally triggered and many of which are time dependent: removing testicular hormones from a male rat on postnatal day 3 results in a behaviorally "asexual" adult; adding testicular hormones to a female rat on the same postnatal day creates a behaviorally "bisexual" adult. Prenatal stress—such as crowding, physical restraint, and bright light—later decreases the adult female rat's fertility and engenders male-like courtship behavior in the adult female guinea pig.

The most satisfying aspect of Hines' work is her emphasis on the many ways in which sex and gender research can go wrong and her insistence on recognizing the complexity of the subject. She particularly warns against generalizing from animal models to human beings. In bird species that rely on song to attract mates, several brain regions in the singing male are larger and more densely synapsed than in the silent female. Some studies suggest that rising testosterone levels during the mating season induce more dendrites in the songbirds' brains, allowing for more complex songs as the season progresses; when the season ends and testosterone levels fall, the dendrites regress. Other studies, however, suggest that birds increase their dendrite number through the act of singing itself—an increasingly complex neural organization both causes and results from the singing behavior. In summarizing what we know about human sex differences, Hines writes that "a relationship between a brain structure and a behavior does not imply, on its own, either a causal relationship or, even if causal, that the behavior is innately determined or cannot be changed."

Hines examines what we know about gonadal hormones and human sexuality, sex differences in children's play, and the relationships among androgens, estrogen, aggression and cognition. In doing so, she finds that human gender identity is more flexible than we may have thought and that the human brain is more responsive to experience than our sexual stereotypes would suggest. Her book is well worth reading.

Dr. Bailey is an instructor in psychiatry at the Johns Hopkins University School of Medicine in Baltimore.