Sex Differences in the Human Corpus Callosum: Myth or Reality?

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Abstract

BISHOP, K.M. AND WAHLSTEN, D. Sex differences in the human corpus callosum: myth or reality? NEUROSCI BIOBEHAV REV 21(5) 581-601 1997.—It has been claimed that the human corpus callosum shows sex differences, and in particular that the splenium (the posterior portion) is larger in women than in men. Data collected before 1910 from cadavers indicate that, on average, males have larger brains than females and that the average size of their corpus callosum is larger. A meta-analysis of 49 studies published since 1980 reveals no significant sex difference in the size or shape of the splenium of the corpus callosum, whether or not an appropriate adjustment is made for brain size using analysis of covariance or linear regression. It is argued that a simple ratio of corpus callosum size to whole brain size is not an appropriate way to analyse the data and can create a false impression of a sex difference in the corpus callosum. The recent studies, most of which used magnetic resonance imaging (MRI), confirm the earlier findings of larger average brain size and overall corpus callosum size for males. The widespread belief that women have a larger splenium than men and consequently think differently is untenable. Causes of and means to avoid such a false impression in future research are discussed. © 1997 Elsevier Science Ltd.

Section snippets

A dispute at the turn of the century

Controversy is no stranger to the corpus callosum. In the 1905 issue of Connecticut Magazine, the anatomist Spitzka announced that the corpus callosum was substantially larger in ten brains from deceased men of eminence than in ‘ordinary’ men. The anatomist Bean [8] was especially impressed by the brain of Professor Joseph Leidy, himself a prominent morphologist who generously donated his body to his colleagues for study. The brain was fairly large (1545 g) but had a corpus callosum of record

Impetus for renewed debate in 1982

The modern history of the dispute dates from the widely publicized report in Science in 1982 of a new claim that the splenium of the corpus callosum shows “sexual dimorphism” [34]. The initial findings were presented in a poster at the 1981 Society for Neuroscience meeting in Los Angeles [35]. Data for 15 male and 13 female brains preserved in formalin were analysed, and the authors noted that the females had a significantly larger splenium area (p = 0.05) and a “visually obvious” more bulbous

Selection of studies

A comprehensive literature search found 49 independent studies that examined sex differences in the CC dating from 1982 to 1994. The search was initiated by conducting computer searches of the literature (MEDLINE, PsycINFO, and Current Contents) using ‘corpus callosum’ and ‘sex differences’ as the search terms. Studies not listed on the computer databases were identified through the bibliographies of papers located in the original searches. The world literature contains more than 49 studies but

Corpus callosum and cognitive function

Our review of a substantial literature on the human CC does not support any sex-related difference in the size or shape of the splenium, whether or not adjustments are made for whole brain or cortex size. Because no significant sex difference is established for the splenium, there is no reason to speculate about its possible contribution to cognitive gender differences. The only consistent significant finding is that adult males have a larger average brain size, a fact that has been known for

Acknowledgements

Preparation of this paper was supported in part by grant OGP 45825 from the Natural Sciences and Engineering Research Council of Canada to D.W. and an NSERC postgraduate scholarship to K.M.B. The authors are grateful to Drs B. Bulman-Fleming, W. Byne, P.E. Cowell, V.H. Denenberg, A. Fausto-Sterling and I.C. McManus for their comments on an earlier draft of this paper.

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