As we have shown, genetic correlations among learning abilities and disabilities are substantial—about .70 on average—which suggests that what they have in common is largely genetic in origin. However, genetic correlations are less than 1.0, which means that genes also contribute to making children better at some abilities than others. In other words, some relatively specialist genes (influencing some abilities but not others) also exist. As mentioned earlier, when DNA research identifies QTLs responsible for genetic influence on reading ability, we predict that most of the QTLs will also be associated with mathematics ability. However, we also predict that some of these QTLs will not be associated with mathematics. Because genetic influence on learning abilities is substantial, such specialist genes contribute importantly to dissociations among learning abilities and disabilities even though most genes are generalists.
Multivariate genetic research also has an interesting story to tell about environmental influences on learning abilities and disabilities. Genetic research distinguishes two types of environmental influences. Those that make family members similar are called shared environment
. The rest, those that do not contribute to resemblance among family members, are called nonshared environment
, and this category also includes error of measurement. Multivariate genetic analyses indicate that shared environmental influences are generalists: Shared environmental correlations among learning and cognitive abilities are as high as genetic correlations. For example, in the two recent studies, the shared-environmental correlation was .74 between reading and mathematics at 7 years (Kovas, Harlaar, Petrill, & Plomin, 2005
), and the average shared-environmental correlation was .86 between five components of mathematics at 10 years (Kovas et al., 2007). An obvious hypothesis that has not yet been rigorously tested is that some monolithic factors such as the family's socioeconomic status or school quality might be responsible for these generalist shared-environmental effects.
In contrast to these generalist effects of shared environment, nonshared environmental effects are specialists: Nonshared environmental correlations are low. For example, in the same two studies, the nonshared environmental correlation was .39 between reading and mathematics at 7 years (Kovas et al., 2005
), and the average nonshared environmental correlation was .24 between five components of mathematics at 10 years (Kovas et al., 2007).
Nearly all research attempting to identify specific sources of nonshared environment has focused on family environments rather than school environments and on personality and behavior problems rather than learning abilities. Nonetheless, such research should be informative for future research that will attempt to identify nonshared environments that affect learning abilities. A meta-analysis of 43 papers relating differential family experience of siblings to differential outcomes concluded that “measured nonshared environmental variables do not account for a substantial portion of nonshared variability” (Turkheimer & Waldron, 2000
, p. 78).
The search for nonshared environments might best begin outside the family. For example, initial research supports the hypothesis that peer influence may be an important candidate for a nonshared environment as siblings make their own individual ways in the world outside their family (Iervolino et al., 2002
). However, peers would not seem to be a likely explanation for why nonshared environmental factors change so much from year to year, nor why nonshared environmental factors differ from one academic subject to another (Kovas, Haworth, Dale, & Plomin, in press
). Perceptions of the environment may be an important direction for research because they are specific to the child. A recent study of 3,000 pairs of 9-year-old twin pairs found that children's perceptions of school experiences were significantly but modestly influenced by genetic factors (20% of the variance), but that most of the variance (65%) was due to nonshared environment (Walker & Plomin, 2006
). However, the problem is that these nonshared environmental experiences hardly relate to nonshared environmental variance in academic achievement.
We also need to consider the possibility that chance contributes to nonshared environment in terms of random noise, idiosyncratic experiences, or the subtle interplay of a concatenation of events. However, chance might only be a label for our current ignorance about the environmental processes by which children—even pairs of MZ twins—in the same family and same classroom come to be so different.
Even though we have a long way to go to understand the nonshared environmental influences that are the source of specialist environments, there are important implications now of thinking about specialist environments in relation to education. Almost all work on school environments focuses on shared environmental factors such as family background and school and teacher quality. However, such shared environmental influences have modest effects and, at least for cognitive abilities, decline sharply in importance from childhood to adolescence (Deary et al., 2006
). Moreover, shared environmental influences act as generalists. More important, and of increasing importance during development, are nonshared environmental influences. As we have described, multivariate genetic research shows that these environmental factors primarily work as specialists contributing to differences in children's performances in different areas. One implication is that educational influences might have their greatest impact on remediating discrepant performances among learning abilities (such as differences in reading and mathematics) and discrepancies between learning abilities and cognitive abilities, which is one way to view the topic of over- and under-achievement.