Genes' Sway Over IQ May Vary With Class
Study: Poor More Affected by Environment
By Rick Weiss
Washington Post Staff Writer
Tuesday, September 2, 2003; Page A01
Back-to-school pop quiz: Why do poor children, and especially black poor children, score lower on average than their middle-class and white counterparts on IQ tests and other measures of cognitive performance?
It is an old and politically sensitive question, and one that has long fueled claims of racism. As highlighted in the controversial 1994 book "The Bell Curve," studies have repeatedly found that people's genes -- and not their environment -- explain most of the differences in IQ among individuals. That has led a few scholars to advance the hotly disputed notion that minorities' lower scores are evidence of genetic inferiority.
Now a groundbreaking study of the interaction among genes, environment and IQ finds that the influence of genes on intelligence is dependent on class. Genes do explain the vast majority of IQ differences among children in wealthier families, the new work shows. But environmental factors -- not genetic deficits -- explain IQ differences among poor minorities.
The results suggest that early childhood assistance programs such as Head Start can help the poor and are worthy of public support. They also suggest that middle-class and wealthy parents need not feel guilty if they don't purchase the latest Lamaze mobile or other expensive gadgets that are pitched as being so important to their children's development.
"How many books are in the home and how good the teacher is may be questions to consider for a middle-class child, but those questions are much more important when we're talking about children raised in abject poverty," said lead researcher Eric Turkheimer, a psychologist at the University of Virginia.
The work, to be published in the November issue of the journal Psychological Science, is part of a new wave of research that embraces a more dynamic view of the relationship between genes and environment. Although older research treated nature and nurture as largely independent and additive factors, and saw people as the sum of their genetic endowments and environmental experiences, the emerging view allows that genes can influence the impact of experiences and experiences can influence the "expression," or activity levels, of genes.
In Turkheimer's study, the impact of genes on IQ varied depending on a child's socioeconomic status (SES), a sociological measure that includes household income and other elements of class and social status.
Until recently, Turkheimer and others said, research had indicated that the "heritability" of IQ -- that is, the degree to which genes can explain the differences in IQ scores -- completely dominated environmental influences. That led some to call into question the value of programs such as Head Start, which are based on the assumption that by improving the childhood environment through extra attention, nutrition and care, a child's intellectual future could be improved.
But it turned out that virtually all those studies on the heritability of IQ had been done on middle-class and wealthy families. Only when Turkheimer tested that assumption in a population of poor and mostly black children did it become clear that, in fact, the influence of genes on IQ was significantly lower in conditions of poverty, where environmental deficits overwhelm genetic potential.
"This paper shows how relevant social class is" to children's ability to reach their genetic potential, said Sandra Scarr, a professor emerita of psychology now living in Hawaii, who did seminal work in behavioral genetics at the University of Virginia.
Specifically, the heritability of IQ at the low end of the wealth spectrum was just 0.10 on a scale of zero to one, while it was 0.72 for families of high socioeconomic status. Conversely, the importance of environmental influences on IQ was four times stronger in the poorest families than in the higher status families.
"This says that above a certain level, where you have a wide array of opportunities, it doesn't get much better" by adding environmental enhancements, Scarr said. "But below a certain level, additional opportunities can have big impacts."
The principle is straightforward and has long been recognized in plants and other simpler organisms. In one famous example, often repeated by evolutionary biologist Richard Lewontin, two genetically identical seeds of corn, planted in very different soil conditions, will grow to very different heights.
Some social psychologists and behavior geneticists have hypothesized that the same must hold true for the relationships linking human genes, socioeconomic status and IQ. Like corn in depleted soil, the thinking goes, minorities and the poor (two categories with so much overlap that researchers find it difficult to tease apart their effects) perform worse not because of their genes but because they are raised in an environment lacking in resources and poisoned by racist attitudes.
"It's a hypothesis that makes a great deal of sense on its face, but has been difficult to study," Scarr said. Difficult, she said, because the best way to study the relative contributions of genes and environment to a human trait is to conduct studies on twins or, in some cases, adopted children. And almost all the twins and adoptees who have been available for study over the years have come from middle-class or higher-class families.
Turkheimer got around that shortage by tapping into data from the now defunct National Collaborative Perinatal Project, which started in the late 1960s. That study, funded by the National Institutes of Health, enrolled nearly 50,000 pregnant women, most of them black and quite poor, in several major U.S. cities. Researchers collected loads of data on the families and gave the children IQ tests seven years later.
Although the study was not designed to study twins, it was so big that many twins were born -- 623 pairs, to be exact, 320 of whom were successfully located by the original researchers and tested for IQ at age 7 in the 1970s. By culling through those test scores and the data on the families' socioeconomic status, Turkheimer was able to conduct one of the first analyses of the role of genes in IQ among the poor.
Twin studies are useful because there are two kinds of twins -- identical twins, which are 100 percent genetically identical, and fraternal twins, which (like other siblings) are 50 percent genetically identical.
Whether twins are identical or fraternal, they share identical prenatal conditions in the womb as they gestate together and they are raised in virtually identical environmental circumstances. That cuts out a major share of environmental differences between the two in any pair.
So when scientists find traits that are more commonly shared between identical twins than between fraternal twins, that suggests the trait is one with a strong genetic basis.
Taking advantage not only of that unique population of children but also of new statistical methods that allowed them to measure complex interactions, Turkheimer and his colleagues -- including University of Minnesota behavioral geneticist Irving Gottesman -- found that the lower a child's socioeconomic status, the less impact genetic inheritance had on IQ.
"It gets away from the pessimistic conclusion that high heritability means you're wasting your money on Head Start," Gottesman said. He suggested that other interventions, including improved prenatal care, would raise IQ even more.
And although IQ remains a controversial measure, criticized by some as being racially biased in itself and a poor reflection of intelligence in the highest sense of the word, Gottesman and others noted that it remains the best predictor today of social and economic success in U.S. society.
Robert Plomin, a behavioral geneticist with the Institute of Psychiatry at King's College of London, who has been seeking genes linked specifically to intelligence, said the results do not undermine the importance of genes.
"In study after study, the evidence is overwhelming that there is a substantial genetic input to IQ," Plomin said. "This doesn't contradict that, but it leads to an interesting possibility that although it's true for the [middle- and upper-class] populations that have been studied . . . it's not going to mean much if you're in an impoverished environment."
Plomin said his own unpublished work involving 4,000 pairs of twins has not produced the same results as Turkheimer's. "We've looked at this for families unemployed, on state support and living in subsidized housing, and we still don't find it, even at that low level" of socioeconomic status, he said.
But, he said, that may simply mean that his population was not as poor as Turkheimer's -- or was benefiting from Britain's superior social safety net.
In fact, the families in Turkheimer's study were very poor, with a median income of $17,000 a year in 1997 dollars. One in five of the mothers was younger than 21, one-third of them were on public assistance, and more than one-third did not have a husband.
Marcus Feldman, a population geneticist at Stanford University who has studied gene-environment interactions, said the next big challenge is to find out what it is about socioeconomic status -- a measure that includes not only income but also parental education and occupational status -- that contributes to IQ, so social programs can more effectively boost those factors.
"SES is a surrogate for something that deserves further study," Feldman said. "A paper like this reemphasizes the importance of psychology and educational psychology and draws us somewhat away from genetics and back into the importance of the social sciences for understanding IQ. This says to me, let's spend the money and find out what it is about SES that makes the difference."
© 2003 The Washington Post Company