Schoenemann, P. Thomas, 1995, "Brain size scaling and body composition in mammals: Implications for the sex difference in brain size in Homo sapiens, " (abstract of paper presented) American Journal of Physical Anthropology, Supplement 19:298.

Abstract

It is well known that brain size scales with body size across large groups of animals, but body size might simply be a proxy for some more important underlying variable. Given that muscle tissue is more intimately associated with brain function than is adipose tissue, it is possible that some estimate of 'fat-free' weight would be a more appropriate scaling parameter for comparing brain size. The possibility that brain size scales more closely with 'fat-free' weight than with total body weight was tested comparatively using Pitts and Bullard's (1968) data, which includes body composition and organ weight data on 49 species in 11 different orders of mammals. Applying Felsenstein's independent comparisons method (limiting possible statistical biasing effects due to phylogeny) indicates that brain size consistently correlates more highly with 'fat-free' weight than it does with 'fat' weight.

This has potentially important implications for the average sex difference in brain size in humans. A difference approaching 1 standard deviation (about 135 g) in brain size has been found in large cadaver studies, as well as numerous studies using cranial capacity as a surrogate variable for brain size. About 100 g of this difference remains even after correcting for total body size. However, correcting for total body size does not correct for sex differences in lean body mass, and this may explain the brain size difference. Females typically average something close to 30% body fat, compared to approximately 15% for males. A survey of the literature on body composition indicates that males and females differ by only ~1.5 standard deviations in total weight, on average, but between 3 and 4 standard deviations in fat-free weight (determined by hydrostatic weighing).

While there are apparently no studies in humans of the correlation between brain size itself and 'fat-free' weight, two studies were found that report correlations between head circumference and 'fat-free' weight. Both studies found significantly larger correlations than have been found between head circumference and body weight. Given: 1) the sex difference in lean body mass and 2) the correlation between head circumference and 'fat-free' weight, much of the sex difference in head circumference in these two samples may be explained by sex differences in the amount of 'fat-free' weight. Suggestions for future research on this issue will be discussed.