When Environment Matters
Prior to Eric Turkheimer’s 2003 paper “Socioeconomic Status Modifies Heritability of IQ in Young Children,” scientists had found that genes account for most of the differences in IQ.
Turkheimer found that although the environmental impact on IQ is very small when you look at families with incomes in the moderate range, the reverse is the case when you look at very low-income families.
Impoverished families were underrepresented in most earlier studies.
Two things made Turkheimer’s project possible: the public release of a large, rich database including an unusually large number of children from families of very low socio-economic status; and advances in statistical methods that use computers to carry out huge numbers of computations very quickly.
Research like Turkheimer’s relies on the difference between fraternal twins, who are as genetically similar as any siblings, and identical twins, who share exactly the same genes.
For traits that are primarily determined by genes, identical twins will show no variation, but fraternal twins will. For traits that are determined by environment, identical twins and fraternal twins will show similar patterns of variation in the trait. For traits that reflect an interaction between genes and environment, identical twins will show somewhat less variation than fraternal twins.
Turkheimer found that for the families in the study at the very bottom of the socioeconomic scale, shared environment accounted for 60% of the variance in IQ; and the contribution of genes was close to zero. Non-shared environment, which includes factors such as gender, accounted for the remainder.
Further up the socio-economic ladder, the effects of environment rapidly declined, and genes took on an increasingly important role.
The research does not identify the specific environmental factors that contribute to the variations in IQ among impoverished children.
“New Thinking on Children, Poverty & IQ,” by Jan Richter