Reply To: Why Are Men More Intelligent Than Women?

Within human population, studies have been conducted to determine whether there is a relationship between brain size and a number of cognitive measures. Anthropologists and psychometricians found correlations, and modern studies began with Van Valen 1974, which suggested a correlation of 0.3. Jensen & Johnson’s 1994 “Race and sex differences in head size and IQ” wrote of the successive literature that:

“In all of the 25 independent studies we have found in the literature, nonzero positive correlations between head measurements and intelligence measurements have been found, all but five with correlations significant beyond the .05 confidence level. The average correlation between various external measures of head size and IQ is close to + .15. Two recent studies have measured brain size per se by means of magnetic resonance imaging (MRI) and found correlations with IQ in the .30 to .40 range (Andreasen et al., 1993; Willerman, Schultz, Rutledge, & Bigler, 1991).”

Nguyen & McDaniel’s 2000 meta-analysis found similarly, as did McDaniel’s 2005 “Big-brained people are smarter: A meta-analysis of the relationship between in vivo brain volume and intelligence”:

“…based on 37 samples across 1530 people, the population correlation was estimated at 0.33…For external head measures, Vernon et al. (2000) reported the population correlation between head size and intelligence to be 0.19. Nguyen and McDaniel (2000) reported population correlations from 0.17 to 0.26 for three different sub-categories of external head size measures.