Genetic and environmental influences on adult intelligence and special mental abilities

Human Biology, Apr 1998 by Bouchard, Thomas J Jr

Importance of Age. The strong age effects in data on unrelated individuals reared together (the disappearance of common environmental influence) are mirrored in the ordinary twin data, where we also see an increase in genetic influence. Figure 3, based on the twin literature broken out into five age categories (McGue et al. 1993), shows these effects graphically.

The change appears to occur at about the time most individuals leave home and begin to function as adults on their own. Similar findings have been reported for the twins reared together being studied in the Minnesota Twin Study of Aging (McGue et al. 1993). A study that has combined data from the adult twins of the Minnesota Study of Aging and the adult twins (reared together) from the SATSA showed that for the age periods 27-50 years and 50-65 years the heritability of IQ was 0.81 and c^sup 2^ was essentially 0 (Finkel et al. 1995). The older Swedish twins (65-85 years), however, showed somewhat of a decline in heritability (0.54), whereas the Minnesota twins did not. More recently, the Swedish group has published data from their study of same-sex twins reared together who are 80 or more years old (McClearn et al. 1997). Only twins with no major cognitive, sensory, or memory impairment were included. For the first principal component of an assessment battery the MZ correlation was 0.75 (N = 110 pairs) and the DZ correlation was 0.38 (N = 130 pairs). The estimated heritability was 0.62. The shared environmental effect of 0.11 was not significantly different from 0 and could be dropped from the model.

Heritability Estimate from Nontwins: Siblings Reared Apart and Together. Teasdale and Owen (1984), who provided one of the adult data points for unrelated individuals in Figure 2, actually reported on five types of sibling correlations: full siblings reared apart, maternal half-siblings reared apart, paternal half-siblings reared apart, unrelated individuals reared together, and full siblings reared together. Their sample was obtained from the Danish adoption register and consisted of only males between the ages of 18 and 26 years. All Danish males, whether fit for military service or not, are required to report to the conscription board and complete a welldeveloped intelligence test (Rasch 1980). Teasdale and Owen's cases were drawn from the conscription board files. As a consequence, their sample is perhaps one of the most representative samples that has ever been used for estimating the heritability of IQ in a population, at least for males. Their correlations and the genetic and environmental parameter estimates are given in Table 3.

Teasdale and Owen (1984) tested for but found no evidence of the effects of nonadditive genetic variance or assortative mating. It is of interest to note that their heritability of 0.96 is the highest value yet reported in the literature. It is often asserted that twin studies (both reared apart and reared together) overestimate the heritability of IQ relative to other designs. Teasdale and Owen disprove the claim. Their group of kinships was well sampled, tested with an excellent instrument, and contained no twins, yet it yielded the highest heritability estimate of IQ ever reported.