INTELLIGENCE 16, 315-328 (1992) Hemisphere Size Asymmetry Predicts Relative Verbal and Nonverbal Intelligence Differently in the Sexes: An MRI Study of Structure-Function Relations LEE WILLERMAN ROBERT SCHULTZ J. NEAL RUTLEDGE ERIN D. BIGLER University of Texas at Austin Hemispheric size asymmetry differences as assessed from MRIs of 39 healthy college students were correlated with prorated WAIS-R Verbal IQ minus Performance IQ and Vocabulary minus Block Design scores within subjects. In men, a relativelylarger left hemisphere predicted better verbal than nonverbal ability,whereas in womena larger left hemisphere predicted relatively better nonverbal than verbal ability. The results were interpreted as providingevidence for sex differences in brain organization. Regional brain-size differences across related species appear to obey the princi- ple of "proper mass" (Jerison, 1973, 1988): Species specializing more in one function than another (e.g., hearing more than vision) have relatively more brain tissue devoted to the more dominant function. The principle is evident in maps of sensory and motor homunculi where cortical area is proportional to the resolution or precision needed for controlling body parts (Geschwind, 1979). Should the principle of proper mass be generally applicable to human cdgnition, more powerful predictions about neuroanatomicai structure-function relations for abil- ities would become possible. Brain size in women is smaller than in men, even after correcting for body size (Ankney, 1992; Gur et al., 1991). Because of no comparable sex difference in general ability, this observation constitutes a challenge to the generalization Robert Schultzis now at the Child StudyCenter, YaleUniversity. J. Neal Rutledgeis also with the Austin RadiologicalAssociation. Erin D. Bigler is now at the Departmentof Psychology, Brigham Young University. We thank Naftali Raz for commentson an earlier version of this article. Correspondence and requests for reprints should be sent to Lee Willerman, Department of Psychology, Universityof Texas at Austin, Austin, TX 78712. 315