Size Matters

Brain structure sizes as functions of cerebral
volume: Leonard et al (2008)

Leonard et al (2008) propose an alternative theory to sex differences in brain structure: it's not the sex, it's the size. In a sample of 200 men and women (mean ages: 21.7 & 21.5), Leonard et al's study found that differences in the corpus callosum, grey/white matter ratios, and cerebrospinal fluid could be better modeled by overal cerebral volume rather than sex (see figure, left).

Specifically, the authors studied differences in the "language areas" of the brain, in an attempt to understand the female advantage in language tasks. The sample did not differ significantly on tests of language ability, parental education, or handedness. The authors note that sex differences in body and brain size are robust, but the relationship between them is not linear: boys show greater cerebral volume before they overtake girls in body size. They conclude that "men and women confront similar cognitive challenges using differently sized neural machinery".

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So, in order to meet the same challenges, smaller-brained people tend to develop more white matter and larger corpus callosi, while larger-brained people develop more grey matter? The idea makes sense (in terms of explaining the available data), but I'm concerned about the graph, which looks more like a splat than a line to me. Still, the main idea about compensatory morphology is consistent with what Cahill's 2006 review said: "neural sex differences can, in some cases, create behavioural sex differences, but might, in other cases, prevent them."

However, this begs the question: do women in the sciences (who one would imagine to have more male-typical brains) tend to be be bigger (or have bigger cerebral volume) than women in female-typical fields? Do men in the arts tend to be smaller? In short, does brain morphology predict field choice?