Race, Evolution, and Behavior

Revamping Social Science 37
height of head) than others. Soon after, K. Pearson (1906) re-examined the
relationship, using the newly developed correlation coefficient, and found a
small positive correlation. This has remained the general observation with
correlations typically ranging from 0.10 to 0.30 (Jensen & Sinha, 1993; Wickett,
Vernon & Lee, 1994; Van Valen, 1974).
Table 2.2 summarizes the results from 32 studies of the relation between
head size and mental ability in normal samples. Clinical samples have been
excluded. The most representative or average correlation has been reported
from those studies providing multiple correlations (e.g. by age and sex or by
adjusting for body size). Corrections for body size have typically not been
included because many studies did not report this statistic although occasionally
they have been used to control for age effects. Double entries have been
eliminated, particularly those emanating from the Collaborative Perinatal
Project (Broman, Nichols, Shaughnessy, & Kennedy, 1987). Also not included
in Table 2.2 are typological studies showing that mentally defective children
have smaller heads than children of normal intelligence (Broman et al., 1987),
while gifted and superior children have larger ones (Fisch, Bilek, Horrobin, &
Chang, 1976; Terman, 1926/1959: 152).
The 32 studies are categorized into 3 sections. Section A sets out the results
of 13 studies that took external head measurements from a total of 43,166
children and adolescents and correlated these with mental ability estimated by
ratings, school grades, and standardized tests. The correlations ranged from
0.11 to 0.35 with an unweighted mean of 0.23 (when weighted by sample size,
0.21). The relationship was found in boys and girls, in whites from Australia,
Europe, and the United States, in blacks from the United States, and in
Amerindians from Guatemala.
Section B sets out the results from 15 studies using external head measurements
from a total of 6,437 adults with intelligence estimated by ratings, university
grades, and standardized tests. The correlations ranged from 0.03 to
0.39 with an unweighted mean of 0.15 (when weighted by sample size, also
0.15). The samples included both sexes, whites from Europe, Canada, and the
United States, and Amerindians and Orientals from North America.
The correlations in Section A and B are low. This is partly because measuring
head size by tape and ignoring skull thickness is not a perfect measure of
brain size and also because intelligence tests are not perfect measures of mental
ability. It is possible to correct the correlations for some of these
unreliabilities. In his review Van Valen (1974) estimated that the true correlation
between head size and intelligence is about 0.30. This was confirmed by
R. Lynn (1990a) in three studies of 9- and 10-year-olds in schools in Northern
Ireland and England measuring head perimeter by tape and intelligence by
standardized tests. Before correction for attenuation due to measurement error,
R. Lynn's correlations were between 0.18 and 0.26; after correction they
ranged from 0.21 to 0.30.