Whether intelligence is more the product of nature or nurture has long fascinated American social scientists and the general public alike. Typically the result is explained as some balance of genetics and environment, but since the early 1970s, researchers have noticed that this scale tends to shift dramatically across social classes. It’s as if nature and nurture play by different rules for rich and poor.
Generally speaking this work has found that genetic variance tends to explain the bulk of IQ scores for advantaged groups, whereas environmental variance plays a larger role for disadvantaged ones. (This line of research draws its results from comparative analyses of identical twins, who share a complete genetic makeup, and fraternal twins or siblings.) In other words, when it comes to intelligence, a comfortable upbringing seems to help nature reach its potential, but an impoverished one seems to interfere at every turn.
Still, other studies have failed to confirm these findings, enough so that scholars continue to wonder. But a strong new analysis published in the journal Psychological Science suggests that the role of genetics in intelligence indeed varies with socioeconomic status—at least in the United States. The data reveal no such pattern in other parts of the developed world, a finding the researchers attribute to “more uniform access” to social programs such as strong education and health care.
“The differences observed across nations might be explained by weaker social safety nets in the U.S. compared to Western Europe and Australia,” the psychologist Elliot Tucker-Drob of the University of Texas at Austin, the paper’s lead author, tells CityLab via email. “While this study did not investigate specific policies or services that might explain the differences … I think that it is fair to say that the causes of the difference are likely to be manifold.”
For this research, Tucker-Drob and the psychologist Timothy Bates of the University of Edinburgh, in Scotland, identified 14 of the most reliable studies ever conducted on the role of nature and nurture on intelligence—eight from the U.S., and six from other countries. Altogether the data set comprised nearly 25,000 pairs of twins or siblings, for roughly 50,000 total individuals. This sort of meta-analysis heightens the statistical power of the research, lending more confidence to its conclusions.
In the American samples, Tucker-Drob and Bates found a significant “gene-by-socioeconomic status” effect—meaning the genetic influence on IQ scores and achievement varied with social class. The chart below shows that genetic variance in intelligence was low for those below the mean socioeconomic status, and steadily rose for those above it. At two standard deviations above the mean, genetics explained about 61 percent of the intelligence outcomes. The role of the environment on intelligence, meanwhile, also tended to decline as social status improved, though that relationship was less statistically significant.
In the non-U.S. samples, the researchers found no significant interaction between genes and socioeconomic status for intelligence outcomes. (They actually reached a conclusion opposite to that of the U.S. in the Netherlands.) The source of the geographical gap might come down to the different types of social programs employed in the U.S. and other parts of the world—specifically those related to literacy, school quality, medical access, income, and upward mobility.
Here’s Tucker-Drob and Bates in their report:
Our results suggest that large-scale genetically informed research that incorporates careful measurement and consideration of both proximal and national social factors may provide a unique key to understanding the impact of specific policies on individual differences in intellectual development and academic achievement.
The U.S. finding, in particular, “resolves an important debate,” according to the researchers. It held up to several potential moderating factors, such as childhood age, various measures of socioeconomic status, and single-versus-composite measures of cognitive ability; it also held up to a robustness check that removed two especially large data samples. They chalk up the inconsistency of previous findings to the “low power” of individual studies.
One thing the study couldn’t determine was whether race, which overlaps considerably with poverty in the U.S., played a role in the findings. But Tucker-Drob says he believes the results reflect social disadvantage, not race or ethnicity, because previous work has reached similar conclusions in “racially homogenous samples.” In other words, even when race is held constant, being poor seems to stifle genetic variation on intelligence.
These results are just the latest scientific insight into how poverty alters the brain. Lab and field tests have found that scarcity imposes huge strains on mental resources, measured at a drop of 13 IQ points. Imaging studies, meanwhile, have revealed a link between income and the surface area of neural regions related to language and executive functioning in children. The new work tracks longer-term influences—but reaches similarly disturbing conclusions.