The brain morphologic characteristics in humans appear to be sensitive to the effects of both age and sex, and converging data suggest that these two variables may interact over the life span to influence brain size.These data should provide a useful context within which to interpret changes in regional brain structure associated with “abnormal” aging.The neurobiological bases for these sex differences in brain aging are not known.The behavioral effects in humans of these sex differences in brain aging are likewise unknown. Sexual dimorphism of the corpus callosum has remained controversial since the original report by de Lacoste-Utamsing and Holloway in 1982, for several reasons measurements have been performed in a variety of ways in different laboratories, in part because published reports frequently do not describe the methodology in detail;despite known age-related changes during both childhood and adulthood, no investigators have explicitly included age-matched subjects; and the size and shape of corpora callosa vary considerably among individuals, requiring large sample sizes to demonstrate significant sex differences.