Evidence for a Genetic Basis for Intelligence
Familial IQ Correlations Advocates of the hereditarian theory base the genetic
hypothesis on the finding that intelligence (at least as measured by IQ tests)
runs in families. For example, the correlation between parents’ and children’s
performance on IQ tests is about .4 (see Bouchard & McCue, 1981, or Kline,
1991, for references on familial correlations in IQ scores). Especially compelling
is the finding that the correlation between the IQ scores of children adopted at
birth and the IQ scores of their biological parents is higher (about .32) than is
the correlation between the children’s IQ scores and the IQ scores of their
adopted parents (about .15) (Horn, Loehlin, & Willerman, 1975). Figure 36.2
provides a table of familial correlations in IQ performance.
Evidence relevant to the genetic hypothesis comes from research on identical
twins reared apart (e.g., Bouchard & McCue, 1981; Shields, 1962). In this situa-
tion, the individuals have virtually the same genes, but grow up in different
environments. The usual finding is that the correlation between the IQ scores of
twins reared apart is about .7, a high correlation. So despite a dissimilarity in
environments,identicaltwinsrearedapartscoreaboutthesameonIQtests.
This correlation is almost as high as the correlation in IQ between identical
twins reared together (about .8) and much higher than the correlation between
biologically unrelated siblings reared together (about .3). Biologically unrelated
siblings reared together share family environments but not genes. So the ines-
capable conclusion seems to be that genes are a primary determinant of intelli-
gence, at least as measured by IQ tests.
Problems with the Evidence Supporting a Genetic Basis for Intelligence The inter-
pretation that the familial IQ correlations support an overpowering influence of
genes on intelligence is problematic, however. The pattern of familial correla-
Figure 36.2
Familial correlations in IQ performance. The source of these correlations is Bouchard and McCue
(1981).
Individual Differences in Cognition 795