Heritability vs Familiality:
Correlation of genetic relationship versus performance on IQ tests

    The data shown above are from the Minnesota Twin Study, performed in the early 1960s. The study measured statistical correlations between performance on IQ tests with degree of genetic relatedness (that is, the heritability of test performance) ranked 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 multiple studies of phenotypic correlations by degree of relatedness irrespective of familiality.

    The data show that performance on IQ tests is heritable in the strict sense: there is a correlation between performance and genetic relatedness. However, phenotypic correlation varies widely within each relatedness class, according to various environmental factors and complex environmental X genetic interactions that are difficult to tease apart.

    When persons with the same genetic correlation (relatedness) are raised together or apart:

1. Identical twins raised together are markedly more similar than those raised apart. This indicates that environment has significant 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 socio-economic environments.

2. When reared together in the same family, pairs of unrelated persons including adoptive children of foster parents, 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 somewhat 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 a range of similarity (0.2 ~ 0.8) that overlaps both that of unrelated persons and twins.