Correlation of genetic relationship versus performance on IQ tests
shown above are from the Minnesota Twin Study, performed
in the early 1960s, which measured statistical correlations
between performance on IQ tests with
degree of genetic relatedness (that is, the heritability of test
performance) rank by degree of familiality
(whether or not they were raised in the same
family). There are three degrees of genetic relationship, R
= 0.0 for unrelated persons, R = 0.5 for either Parent
x Child or Sibling x Sibling (including dizygotic
twins), and R = 1.0 for monozygotic identical
twins. The data show that related persons have more similar IQ test scores than do
unrelated persons, and that the similarity increases with degree
of relatedness. The first figure below aggregates all phenotypic
correlations by degree of degree of relatedness irrespective of
1. Identical twins raised
together are markedly more similar than those raised apart. This
indicates that environment has
influence on IQ test scores. This is further confounded
by the likelihood that identical twins, even when adopted into
separate families, are likely to be placed and raised in similar
2. When reared in the same family, pairs of unrelated persons, sibs, and identical twins are on average more similar than those raised apart. This indicates that familiality has significant influence on IQ test scores.
3. Two-egg twins of like sex are more similar than those of opposite sex: the former are more likely to be treated similarly than the latter, as are one-egg twins (always of the same sex).
4. Parent-child pairs show an extreme range of similarity (0.2 ~ 0.8) that markedly overlaps that of unrelated persons and twins.