More about the effect of socioeconomic status on general intelligence

We've  recently read in the WISCR Hebrew manual about the connection between socioeconomic status (SES) and general intelligence.

Another evidence for this connection comes from adoption studies.  Adoption usually transfers children from low to higher SES, because an adoptive family's  SES  is usually at least average, while the SES of a family who gives its child up for abortion is usually low.  Adopted children usually have at least 12 IQ points more than siblings who stay with the biological parents or children adopted by low SES parents.

Furthermore, during the summer vacation, children from low SES homes lose IQ points (and academic skills), whereas children from high SES homes (homes that are in the top quintile) gain IQ points and academic skills.  This may be due to the enriching activities that high SES parents are able to expose their children to.

This finding attests to the importance of implementing good quality summer programs for children from low SES homes. 

Another interesting evidence for the influence of SES on general intelligence comes from twin studies,  Turkheimer and his colleagues studied 319 pairs of twins, 114 of whom were monozygotic (identical twins, who are genetically nearly identical) and 205 of whom were dizygotic (fraternal twins, essentially, two ordinary siblings who happen to be born at the same time, and share only about 50% of their genes).  The twins took the WISC test at the age of seven.  The researchers wanted to know the relative influence of environment versus genetics on the twins'  IQ scores.  If genetics has a strong influence on the IQ scores, the correlation of the identical twins' IQ scores will be higher than the correlation of the fraternal twins' IQ scores.  If the twins' shared environment  (the food they eat, the family outings they share, books they have at home, the level of conversation they have as a family, the parent's approach to education etc.) has a strong influence on the IQ scores, there will not be a difference between the IQ scores correlations of the identical and fraternal twins.

Surprisingly, the relative influence of genetics and shared environment varies in different SESs! For families at the lowest levels of SES, shared environment accounted for almost all of the variation in IQ, with genes accounting for practically none. As SES increased, the contribution of shared environment diminished and the contribution of genes increased, crossing in lower middle-class families. Finally, in the most socioeconomically advantaged families (who were not wealthy), practically all of the variation in IQ was accounted for by genes, and almost none was accounted for by shared environment.

Here is one possible explanation for this:  it's possible that the environment of children raised at lowest levels of SES does not supply them with lots of enriching and varied stimuli or activities.  Then, every enriching opportunity the child gets can gives him a chance to develop and realize his genetic mental potential.   High SES children already live in an enriching environment.  They exhaust  or realize  their full genetic mental potential.  Differences   in the enrichment and nurturance families of high SES supply don't make that much of a difference to their children's IQ scores.

This means that good interventions can be very helpful to children from low SES homes.  The best prekindergarten programs for lower SES children have a substantial effect on IQ, but this typically fades by late elementary school, perhaps because the environments of the children do not remain enriched. There are two exceptions to the rule that prekindergarten programs have little effect on later IQ. Both are characterized by having placed children in average or above-average elementary schools following the prekindergarten interventions. Children in the Milwaukee Project program had an average IQ 10 points higher than those of controls when they were adolescents. Children in the intensive Abecedarian prekindergarten program had IQs 4.5 points higher than those of controls when they were 21 years old.

Nisbett, R. E., Aronson, J., Blair, C., Dickens, W., Flynn, J., Halpern, D. F., & Turkheimer, E. (2012). Intelligence: new findings and theoretical developments.American psychologist, 67(2), 130.‏

Turkheimer, E., Haley, A., Waldron, M., D'Onofrio, B., & Gottesman, I. I. (2003). Socioeconomic status modifies heritability of IQ in young children.Psychological science, 14(6), 623-628.‏