Size Matters: Cerebral Volume Inﬂuences Sex Differences in Neuroanatomy
Christiana M. Leonard1, Stephen Towler1, Suzanne Welcome2, Laura K. Halderman2, Ron Otto3, Mark A. Eckert4 and Christine Chiarello2 1 University of Florida, Gainesville, FL 32601, USA, 2University of California, 3Diagnostic Imaging Center, Riverside, CA 92517, USA and 4Medical University of South Carolina, Charleston, SC 29403, USA
Biological and behavioral differences between the sexes range from obvious to subtle or nonexistent. Neuroanatomical differences are particularly controversial, perhaps due to the implication that they might account for behavioral differences. In this sample of 200 men and women, large effect sizes (Cohen’s d > 0.8) were found for sex differences in total cerebral gray and white matter, cerebellum, and gray matter proportion (women had a higher proportion of gray matter). The only one of these sex differences that survived adjustment for the effect of cerebral volume was gray matter proportion. Individual differences in cerebral volume accounted for 21% of the difference in gray matter proportion, while sex accounted for an additional 4%. The relative size of the corpus callosum was 5% larger in women, but this difference was completely explained by a negative relationship between relative callosal size and cerebral volume. In agreement with Jancke et al., individuals with higher cerebral volume tended to have smaller corpora callosa. There were few sex differences in the size of structures in Broca’s and Wernicke’s area. We conclude that individual differences in brain volume, in both men and women, account for apparent sex differences in relative size. Keywords: asymmetry, corpus callosum, gray matter, human, planum temporale Introduction Biological and behavioral differences between the sexes range from obvious to subtle or nonexistent. Among the most controversial have been neuroanatomical differences because of the implication that behavioral differences could be due to fundamental differences in brain organization or neural potential (Halpern et al. 2007). The current study, part of the Biological Substrates for Language Project, affords the opportunity to explore this issue in 200 college-aged men and women. Reported sex differences in neuroanatomy include highly reliable differences in cerebral volume (Willerman et al. 1991; Andreasen et al. 1993), cerebellar volume (Filipek et al. 1994), and gray matter proportion (Gur et al. 1999; Allen et al. 2003) and less reliable differences in the asymmetries (reviewed by Beaton 1997; Shapleske et al. 1999) and proportional sizes of various language areas (Harasty et al. 1997; Rademacher et al. 2001; Knaus et al. 2006; Vadlamudi et al. 2006). The reports of sex differences in shape and proportional size of the corpus callosum are relatively more consistent (Witelson 1989; Allen et al. 1991; Steinmetz et al. 1992; Driesen and Raz 1995; Bishop and Wahlsten 1997; Davatzikos and Resnick 1998; Luders et al. 2003). We were particularly intrigued by the possibility that certain structures occupy proportionally more brain volume in women than men. This is a signiﬁcant claim given the strong relation
that normally exists between subregion size and total volume. After an exhaustive study of 131 species of mammals, Finlay and Darlington concluded that ‘‘the most likely brain alteration resulting from selection for any behavioral ability may be coordinated enlargements of the entire nonolfactory brain’’ (Finlay and Darlington 1995, p. 1578). A demonstration that selected structures were proportionally increased in women would suggest that sex differences in behavior had been accompanied by a biologically unusual targeted enlargement of speciﬁc regions or networks. Most notably, these claims have been made for gray matter volume, corpus callosum, and perisylvian...