Correlation of genetic relationship versus performance on IQ tests
shown above are taken from the Minnesota Twin Study,
performed in the early 1960s, which measured statistical
correlations between performance on IQ tests with
degree of genetic relatedness, modified by degree of familiality
(whether or not they were raised in the same family). There
are three degrees of genetic relationships, 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 familiality.
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 (similarity of family background) 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.