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.
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