Genetic and environmental influences on adult intelligence and special mental abilities

Human Biology, Apr 1998 by Bouchard, Thomas J Jr

To represent the correlation between dizygotic (DZ) twins reared apart (r^sub DZA^), who share half their genes in common by descent, the correlation between G's in Figure lb would become 0.5 and the equation would read r^sub DZA^ = 0.5h^sup 2^. The correlation between DZ twins reared apart directly estimates one-half the narrow heritability. This version of the model makes the assumption that there is no nonadditive genetic variance and no assortative mating on the genetic component of the trait.

Dizygotic twins (reared together or apart) share only 0.25% of their dominance variance (one component of the nonadditive genetic variance), whereas MZ twins share all of the dominance variance; consequently, if the DZ correlation is reliably less than half the MZ correlation, this is presumptive evidence of nonadditive genetic variance. Interactions between loci (epistasis) can also create nonadditive genetic variance and cause DZ twins to be less than half as similar as MZ twins. Dominance and epistasis are often lumped together and simply called nonadditive sources of genetic variance.

Figure 1c shows the model for the correlation obtained from unrelated individuals reared together (r^sub URT^). The correlation in this instance (r^sub URT^ = c2) estimates what is called common environmental influence. Figure ld shows the components of the correlation for MZ twins reared together (r^sub MZT^). The correlation in this instance is confounded (r^sub MZT^ = h2 c2) and estimates the sum of the genetic and common environmental variance. It simply combines the models in Figure 1b and 1c. The correlation for DZ twins reared together would be r^sub DZT^ = 0.5h^sup 2^ c^sup 2^. The twin literature is full of heritability estimates based on the formula 2(r^sub MZT^ - r^sub DZT^). This equation, called the Falconer heritability formula (Falconer 1990), assumes that c2 is the same for MZ and DZ twins reared together-the equal trait-relevant environment assumption-and that all genetic influence is additive.

Assortative mating on the genetic component of a trait creates a greater similarity between first-degree relatives than would be found under random mating and thus would make the DZ correlation more similar than half the MZ correlation. There is considerable evidence for assortative mating for IQ and somewhat less evidence for assortative mating for special mental abilities. I do not discuss the complexities of assortative mating for IQ because of a lack of space. Jensen (1978) provided a thorough discussion of this topic.

When data from several kinships are gathered, a variety of model fitting procedures are available to efficiently estimate the underlying genetic and environmental parameters and to statistically test various hypotheses (e.g., Does the addition of a parameter for nonadditive genetic variance result in a significantly better fit or will additive variance suffice? Can the pattern in the data be explained by environmental factors, or does a genetic factor have to be postulated to generate an adequate fit?) (Neale 1995; Neale and Cardon 1992).