One of the hardiest generalizations that has emerged from studies of sex differences in intellectual functioning is that American and European men usually outperform women on tests of spatial reasoning.1 Those who believe that this difference is due partly to biologic influences attribute the effect either to the direct action of gonadal steroids on the central nervous system — during embryogenesis or at puberty — or to sex-linked factors controlling the development of the cerebral cortex more directly. A paper by Hier and Crowley in this issue2 addresses the validity of the first of these mechanisms and reports that men . . .