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, 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 familiality.

    This indicates that performance on IQ tests is heritable, in the strict sense that there is a correlation between performance and genetic relatedness. However, there is a wide range of phenotypic correlations within each relatedness class, according to various environmental factors and complex environmental X genetic interactions that are difficult to separate.

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 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.