Download Eric Turkheimer

Eric Turkheimer
“Socioeconomic Status Modifies Heritability of IQ in Young Children. ” (Nov. 2003)
Nature or nurture?
If you're poor, it's not easy to bloom where you are planted.
By Christine Parker Martin (Foreign Affairs '92)
Posted 2/17/05
Think back … way back …to grade school, when you were given a Styrofoam cup, potting soil
and a bean. You took care to deposit your bean in the rich soil and tenderly watered it. Then,
names were picked from a hat to decide which cups would get a spot on the windowsill,
relegating the others to a shelf inside a cupboard. You might remember the outcome of this
simple biology experiment, particularly if your bean plant resided in the dark cupboard.
As with bean plants, the development of human traits involves both nature (genes) and nurture
(environment). Psychology professor Eric Turkheimer demonstrated this phenomenon as it
applies to IQ in a landmark twin study published last year in Psychological Science.
Turkheimer’s findings diverge from earlier nature/nurture IQ studies, which suggested genes are
nearly all-important in determining differences in human intelligence and consequently led to
heated debate as to whether publicly funded childhood assistance programs like Head Start can
make a difference.
“We found that for the poorest twins, IQ seemed to be determined almost exclusively by their
socioeconomic status, which is to say their impoverished environment. Yet, for the best-off
families, genes are the most important factor to determining IQ, with environment playing a much
less important role,” Turkheimer explained.
Turkheimer’s findings may seem intuitive. After all, the bean plant devoid of light not
surprisingly emerges stunted. And the toy industry is capitalizing on this idea with best-selling
enrichment products like Baby Einstein™ videos and black, white and red mobiles. Moreover,
who hasn’t heard such anecdotal evidence as the inner-city kid who beat all odds and wound up at
Harvard thanks to a teacher who provided nurturing, stability and motivation?
Yet, earlier nature/nurture IQ studies repeatedly demonstrated that people’s genes—not
environment—account for variability in individual IQ. And Turkheimer acknowledges, too, the
undeniable importance of genes to human traits, including IQ. “We often joke in behavior
genetics that everybody is an environmentalist until they have their second child,” he said.
But the research as it stood didn’t satisfy Turkheimer, who felt previous studies told “too simple a
story” because lower-income and impoverished families were typically, yet unintentionally,
underrepresented in such studies. Said Turkheimer: “I’m a clinical psychologist, and I’ve seen
and tested people raised in poverty whom I knew from observation had suppressed IQs because
of their poverty.”
Turkheimer’s study differed from previous twin IQ studies in two important ways. First, he
identified a data source comprised of over 600 twin pairs, of which a substantial proportion
represented families living near or below the poverty level. Second, Turkheimer relied on fairly
recent statistical advances that made it possible to determine the importance of genes as a
function of socioeconomic status.
The study results show that in the most impoverished families, hereditability of IQ is essentially
zero, with environment accounting for almost 60 percent of the differences in IQ among
individuals. The impact of environment declines as socioeconomic level improves, playing a
nominal role in the most affluent families, for which virtually all variability in IQ is attributed to
genes.
The study suggests that specific minimal environmental conditions are necessary for a person’s
genetic potential to be expressed. Socioeconomic status is a complex variable, and Turkheimer
doesn’t identify these factors, such as prenatal care, nutrition, income or parental involvement, for
example, or suggest their relative importance to the results. “I think it’s the accumulation of
many, many small things that together make poverty, rather than any one thing that matters
most,” he speculated.
Turkheimer recently replicated the study using a different twin sample, a necessary step before he
or others can begin to try to understand the phenomenon in detail and look at such questions as
“What kinds of intelligence and abilities, aside from IQ, are particularly sensitive to
environment?” and, conversely, “What kinds of environmental differences are particularly
important to intelligence and abilities?”
Yet even without all the answers, Turkheimer is satisfied that his study now provides a theoretical
framework in which it would be reasonable to expect that programs like Head Start might work.
Noted Turkheimer: “It suggests that if you’re going to work with people’s environment to try and
increase IQ, then the place to invest your money is in taking people in really bad environments
and making them OK, rather than taking people in pretty good environments and making it
better.”