Nature or nurture?

Eric Turkheimer’s IQ study shows that if you’re poor, it’s not easy to bloom where you’re planted.

By Christine Parker Martin (Foreign Affairs '92)

Photo by Jack Mellott.

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.”