Correlation of genetic relationship versus performance on IQ tests

    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. This indicates that IQ is heritable. However, there is a wide range of phenotypic correlations, according to various environmental factors and 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 if adopted into separate families are still likely to be raised in similar socio-economic environments.
2. When reared in the same family, pairs of unrelated persons, sibs, and identical twins are 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.