While the genetic influence on intelligence has been well studied, less is known about school achievement and the genetic/environmental links between intelligence and school achievement. Previous work has also struggled with potential measurement and sampling bias. This study used national data from large Dutch and English samples to address some of these concerns.
The total sample included nearly 174,000 11 year old children in England and 166,000 children in the Netherlands. Intelligence was measured using the Cognitive Ability Test for the English sample while achievement was assessed through schools’ standard school performance tests. Intelligence assessment for the Dutch sample of children ages 8, 10, and 12 came from two tests of nonverbal intelligence while school achievement was measured using scores on arithmetic and language. A “twin extraction” method was used to find twins from the national datasets based on their name and date of birth. Since monozygotic versus dizygotic twins could not be determined through these ratings, the authors used same and opposite sex twin status for their statistical models.
In the English dataset, the heritability of intelligence was found to be high at 70%. The heritability of overall achievement was also found to be high at 75% and ranged from a high of 81% for English to a low of 51% for science. The Dutch dataset found increasing heritability of intelligence between age 8 (43%) and 12 (67%), although at age 10 there was a low heritability of 24% with a shared environmental influence of 29%. This result is generally consistent with other studies that show that the genetic influence on intelligence tends to increase with age. The heritability of school achievement in the Dutch sample ranged from 36% to 74% depending on the age and subject.
As expected, intelligence and achievement were highly correlated with the association somewhat higher in England. This association was related strongly to genetic factors in the English sample and moderately so with the Dutch data. Shared environmental influences explained much of the remaining variance related to the intelligence/achievement link in both samples.
The authors concluded that the known association between intelligence and school achievement is largely accounted for by common genetic factors, although shared environmental factors (things like parenting influences) also played important roles for many subjects and should not be overlooked.
The heritability estimates for intelligence in this study are somewhat above the approximately 50% found in other papers. Some of the differences found between the English and Dutch samples were hypothesized to be related to the different instruments that were used and the Dutch sample having used a greater percentage of children from lower socioeconomic backgrounds.
Finally, it is important to point out that this study was done in two developed countries. As such, the magnitude of the genetic effects could be higher than in developing countries where there are huge discrepancies in opportunity and where sadly many bright children have little chance to show their potential in educational settings.
Calvin CM, et al. Multivariate genetic analyses of cognition and academic achievement from two population samples of 174,000 and 166,00 school children. Behav Genetics 2012; 42:699-710.