The idea of brain sex has acquired a cultural valence and resonance that goes far beyond the scientific evidence that supports it. Scientists use the term in three basic ways measurable anatomy, brain physiology, and behavioral differences attributed to brain function. In a classic study, behavioral biologists Fernando Nottebohm and Arthur Arnold showed that the specific region of the canary brain. In the late 1950s a germinal paper appeared that expanded the concept that fetal hormones affected the development of reproductive physiology, to the more general idea that fetal hormones “organized” brain development in ways that might affect a wide variety of sex-diff erentiated behaviors. The human brain, however, develops slowly but continuously throughout fetal development. In human newborns many parts of the brain start offwith a small density of synapses. The size of the diff erences is considerably smaller than differences found in brain regions governing reproduction.