IQ scores

Heritability as a Phenotype x Genotype correlation

    Consider a set  on educational test scores for six children (C) born to one set of parents and adopted by another set. The mean score of the birth parents (B) (calculated as the mid-parent value) is 95. The average score of the adoptive parents (A) is 115, 20 points higher than that of the birth parents. After adoption, the children are provided with an "enriched" educational environment. Upon testing, they achieve the same average score (115) as their adoptive parents.

    What does this tell us about the heritability of scores? Note that each of the children is exactly 20 points higher than the average of the birth parents: the correlation r (and thus the heritability h2) between them is therefore rBxC = 1.0. The adopted children achieve exactly the same mean score as the adoptive parents. However, the range of the difference is scattered from -8 to +6 points: the correlation between them is rCxA ~ 0. [The correlation between birth and adoptive parents is also  rBxA ~ 0, which shows that the adoptive parents have not selected children based on expected performance].

    That is, a trait that is perfectly heritable may also be perfectly changeable by a modification of the environment
. Modified the environment increases the trait mean, but does not improve any particular individual in a specific way.

Numerical example after Griffiths et al. (2002); All text material ©2014 by Steven M. Carr