There is a significant positive correlation between intelligence and brain size, whether measured directly by neuroimaging or indirectly by cranial volume. Both intelligence and brain size are highly heritable, indicating the importance of genes. Aside from the cranium, other external anatomy could correlate with intelligence. Although some of these structures may appear to have no association with the brain (and intelligence), they in fact share a common developmental and genetic origin. It is possible that these non-obvious traits are differentially distributed across groups as well as individuals. Given the widespread effects of genes and hormones on phenotypic development, it is possible that there are a number of non-obvious human structures that correlate with brain structure and cognitive ability.
Key Words: Cognitive ability; Craniofacial anatomy; Individual and group differences; Myopia; Testosterone.
A man finds room in the few square inches of his face for the traits of all his ancestors; for the expression of all his history, and his wants.
Ralph Waldo Emerson: Conduct of Life
Numerous studies have demonstrated a positive correlation between brain size and cognitive ability (Rushton & Jensen, 2005). Positive correlations of intelligence have been found with indirect measures such as cranial volume and head circumference (r= 0.2), which were later confirmed with in vivo measurements using noninvasive imaging technology (r = 0.4; Rushton & Jensen, 2005).
Furthermore, the sizes of brain regions implicated in cognitive ability have been positively correlated with intelligence (Colom et al., 2006; Frangou et al., 2004; Toga & Thompson, 2005). For example, the frontal cortex is associated with such higher cognitive functions as working memory, attention, planning and language skills. The volume of frontal cortex gray matter is correlated with intelligence. Higher g (general factor of intelligence) in tests is correlated with greater volume of brain areas, such as gray matter of temporal and prefrontal cortex (Jung & Haier, 2007). Twin and sibling studies have demonstrated a high heritability for brain volume (0.66-0.97) and intelligence (0.6 to 0.8; Bouchard & McGue, 2003; Peper et al., 2005). Thus, the high heritability of both brain volume and cognitive ability and their association strongly suggests that a common set of genes mediates the expression of both structure and function.
The correlation between brain size and cognitive ability has been observed irrespective of sex and race and persists when body weight and height are taken into account (Rushton, 1995; Rushton & Jensen, 2005). Furthermore, racial differences in brain size parallel racial differences in cognitive ability. The three most thoroughly studied races are Blacks of African origin, Whites of European origin and East Asians from China, Korea and Japan. Based on measurement of brains at autopsy, endocranial volume and head size, aggregate mean differences in brain volume (cm^sup 3^) are observed: African = 1,267; European = 1,347 and Asian = 1,364 (Rushton & Jensen, 2008). The within-race standard deviation for brain size has been reported as 146 grams for Whites and 144 grams for Blacks (Ho et al., 1980). Note that the difference of the means is 17 cm^sup 3^ between East Asians and Europeans and 80 cm^sup 3^ between Europeans and Africans. Although there is extensive overlap of volumes between Asians and Europeans, the main findings are the robust differences between Asians and Africans and between Europeans and Africans.
Group differences in brain volume parallel those in IQ. Sub-Saharan Africans have a mean IQ of 70, Europeans have a mean of 100 and East Asians have a mean of 103 (Lynn & Vanhanen, 2002). The group differences in IQ persist across national borders. Although brain size and cognitive ability are significantly correlated and both vary by race, cognitive ability is by no way fixed. …