Physiologic differences of cerebral blood flow 6 and glucose utilization between male and female brains 7 likely bear a relation to the morphologic differences described above. The finding of fewer cortical neurons with greater dendritic arborization in females than in males may explain in part women s greater vulnerability for dementing illnesses compared to men 8,9,10 . Also, the smaller total number of cortical neurons present in females could restrict the functional reserves available. The data additionally support the hypothesis that, during gestation, the size of the overproduced and secondarily reduced cortical nerve cell population 11 may play a role in defining the extent of functional deficits resulting from early brain injury. Thus, the retention of a greater percent of nerve cells produced during fetal development in males leading to more cortical neurons later in life, may contribute to the several fold greater incidence of learning difficulties in boys compared to girls 12,13 .
Ongoing research is aimed at defining more precisely the gender difference in neuropil.
Because neuropil is constituted by several tissue elements other than dendrites, we are currently evaluating a possible contribution by pyramidal cell size and astrocyte volume differences to the neuropil estimates in our two gender groups. In addition to global gender differences of the human cerebral cortex, we have identified specific regions and functional types of cortex defined by Benson 14 with particularly large gender differences. Specifically, the gender differences for neuropil estimates are most prominent in types of cortex involved in association functions while primary motor and visual cortex and limbic areas are characterized by smaller, non-significant differences 3 . The analysis of such regional and hemisphere side differences is in progress. The finding of significant gender differences in human brains carries an unfortunate potential for abuse.