Genes and Social Class
A new twins study underscores the power of the environment
BY ROB WATERS
In recent years, there's been a fundamental shift in the old nature/nurture debate most of us started hearing about in high school. A third wave of research is challenging the traditional view of genes and environment as independent actors, each exerting a separate influence on behavior and personality. Instead, what's emerging is a picture of a subtle but dynamic dance between people's genetic endowment and their environment: genes influence the way life events are experienced and experiences affect the way genes are expressed. It's not a question of nature versus nurture, but rather "nature via nurture," in the words of David Lykken, a noted University of Minnesota behavioral geneticist.
Two new studies are prime examples of this third-wave research. The first is a groundbreaking study on the effect of genes and environment on children's IQs being published in the November Psychological Science. Previous studies have consistently found that IQ scores are largely a function of genes and that children's environments count for little. That finding was the basis for the controversial--some would say racist--argument made in the 1994 book The Bell Curve that the low IQ scores of minorities, especially African Americans, were evidence of genetic inferiority.
On a large scale, such arguments have serious implications. If the poor test scores of low-income children are mostly a product of defective genes, not impoverished environments, then programs like Head Start are misguided wastes of money. Likewise, psychotherapy, in the view of biologically minded psychiatrists, can be of little value if the real need is correcting chemical imbalances.
The new study, led by Eric Turkheimer, a genetic researcher at the University of Virginia, relied on the standard technique for assessing the heritability of a trait: it looked at twins. But for the first time, Turkheimer and his team included large numbers of poor families and found that, for those children, the effect of their environments played a much bigger role than their genes in determining IQ.
For years, scientists seeking to understand the influence of genes on traits like intelligence have studied the differences between identical and fraternal twins. Identical twins are like genetic clones: they have exactly the same genes. Fraternal twins have genes that are 50 percent identical, like ordinary siblings. Since both types of twins share the same prenatal and family environments, researchers assume that environmental influences on them are essentially the same and attribute differences between them to their genes.
The problem with this methodology is that most studies of twins have drawn almost exclusively from the middle class and wealthy families that commonly get enrolled into university studies. Turkheimer and his colleagues counteracted this bias by using data from the National Collaborative Perinatal Project, a large government study that began in the late 1950s. Before it was discontinued, it enrolled nearly 50,000 pregnant women, mostly black and poor, and collected data, including IQ tests, from the women and their children.
Because the study was so large, more than 600 pairs of twins were born to the enrolled mothers, and 320 of the twins were given IQ tests at age 7. This 40-year-old data enabled Turkheimer to analyze, for the first time, the roles played by genes and environment in determining the IQ of poor children. Using new statistical methods, he and his colleagues found that, "In impoverished families, 60 percent of the variance in IQ is accounted for by the shared environment and the contribution of genes is close to zero. In affluent families, the result is almost exactly the reverse." The point, says Turkheimer, isn't that genes don't influence IQ in poor families, but rather that it takes an adequate environment for people's genetic potential to be expressed. …