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

Abstract

Abstract: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. Keywords: allometry, brain size, effect size, literature review, meta-analysis, morphometry, ratio measures, statistical power Article:IF men and women think differently, their brains must also differ in some way. This holds true, even if the difference emanates largely from experience, because experience changes the brain (39,114). The size of cognitive gender differences has never been very large (58,69), and the difference has almost vanished for several abilities in more recently published reports (47,70). Of course, not all measures of behavior show small sex differences. For example, in the USA, boys possess considerably more knowledge about electronics, automobiles and machinery (58). Among 21 indicators of sexual behavior and sexuality, two (attitude towards casual intercourse and incidence of masturbation) exhibit male-female differences greater than 0.8 standard deviation (95), which is the conventional criterion for a large group difference in psychological research (27), although most (18 out of 21) show differences less than 0.5 standard deviation (SD). However, on tests of cognitive abilities, sex differences are usually small (58). In a review of 287 effect sizes for spatial visualization abilities (118), the average sex difference was 0.37 SD and only one kind of test (mental rotation) met the criterion for even a moderate effect size. For 254 effect sizes involving mathematical performance in 100 published studies, the male average exceeded the female average by only 0.2 SD, but for studies in which the samples were drawn from the general population, females scored slightly better than males (70). Given such small cognitive gender differences, it seems likely that only the most sensitive and refined neurological techniques will be able to locate the relevant difference in brain tissue.Neuroanatomists have scrutinized thousands of pre...

Schizophrenia, neurodevelopment and corpus callosum

“…Interestingly, these are the regions where axons interconnecting the prefrontal cortex cross the midline 98 ( Figure 1). The study mentioned 83 concurs with the report of a reduced CC size in women 76 in drawing attention to sex-related differences in the neuropathology of S. Nevertheless, given the long history of controversial results with respect to sex differences in CC size, 96 the data mentioned must be considered with prudence. It should be added that counting callosal axons in A = acute, B = boys, C = chronic, ES = early onset S (o35 y), F = females, F>M = more numerous females, G = girls, lh = left handed, LS = late onset (>50 y), SM = males, pS = at risk for schizophrenia, rh = right handed, S = schizophrenics, SPD = schizotypal personality disorder.…”

“…Furthermore, CC size should probably be corrected for total brain size, but although several methods exist, none is fully satisfactory. 96 Also, if the decreased CC is due to loss of callosal axons (below) and this significantly contributes to decreasing brain size (for example, by reducing the volume of the neuropil, that of the white matter, or of the cortical neurons), the correction would, in fact, tend to obscure the finding. Finally, the studies exploring regional differences in CC size are particularly difficult to interpret since methods of CC partitioning vary across studies (reviewed in Bishop and Wahlstein 96 ), including in S research, and the size of the CC partitions can be influenced by CC shape.…”

Structural changes of the corpus callosum in tinnitus

“…There is not much reason to assume that the CC functions in a predominantly inhibitory fashion in males and a predominantly excitatory fashion in females. While some researchers have claimed that the female CC, if scaled relative to whole brain size, is larger and thus facilitates transfer of information between the hemispheres (Oka et al, 1999; Mitchell et al, 2003), others have not found a difference (Bishop and Wahlsten, 1997; Luders et al, 2006; Zarei et al, 2006). Thus, there is little reason to speculate on the possibility of males and females differing with regard to the relation of tinnitus to callosal structure and function.…”

Single-Gene Influences on Brain and Behavior

“…When a new claim is made of weak allele association or linkage with some other measure (such as IQ), experience should caution us against premature enthusiasm until the result is replicated adequately and survives meta-analysis. Otherwise, there arises a serious risk that false claims will mislead public discourse, as allegations of sex-based differences in the human brain (Bishop & Wahlsten 1997) and an alleged relation between serotonin metabolism and impulsive violence (Balaban et al 1996) have already done.…”