Throughout the animal kingdom, nervous systems differ between sexes. Sexual dimorphisms in neuroanatomical, physiological, and neurochemical parameters are reflected by sex-specific forms of behavior, in particular with regard to reproduction, and sex differences in neuroendocrine regulation. The present volume of Biomedical Reviews is mainly concerned with sex differences in the mammalian and human brain. However, it should at least be mentioned that the most convincing correlations between

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... roanatomical and behavioral data have been provided by studies of the sexual differentiation of song control systems in bird species, in which the males sing and the females do not. In mammals, research used to locus on the hypothalamus commonly seen as the center for control of reproductive behavior and integration of hormonal and neural responses of the organism. We have therefore asked A Mafsumofo and %. /*ra/ to review the evidence for the hypothesis that sexually dimorphic functions of the hypothalamus are indeed based on a sex-specific neural circuitry. Notwithstanding the central role of the hypothalamus, it is important to note that more and more, often subtle, anatomical and/or functional sex differences have been and continue to be detected in other areas of the central nervous system. Of particular interest are sex differences in neural systems than are known