Do Genes Predict Intelligence? In America, It Depends on Your Class

There’s a longstanding academic debate about the genetics of intelligence. On the one side is the “hereditarian” camp, which cites a vast amount of research showing a strong link between genes and intelligence. This group can point to persuasive twin studies showing that, by the time children are 17 years old, their genetics explain approximately 66 percent of the variation in intelligence. To the extent we can measure smarts, what we measure is a factor largely dictated by the double helices in our cells.

On the other side is the “sociological” camp. These scientists tend to view differences in intelligence as primarily rooted in environmental factors, whether it’s the number of books in the home or the quality of the classroom. They cite research showing that many children suffering from severe IQ deficits can recover when placed in more enriching environments. Their genes haven’t changed, but their cognitive scores have soared.

These seem like contradictory positions, irreconcilable descriptions of the mind. However, when science provides evidence of two opposing theories, it’s usually a sign that something more subtle is going on. And this leads us to the Scarr-Rowe hypothesis, an idea developed by Sanda Scarr in the early 1970s and replicated by David Rowe in 1999. It’s a simple conjecture, at least in outline: according to the Scarr-Rowe hypothesis, the influence of genetics on intelligence depends on the socioeconomic status of the child. In particular, the genetic influence is suppressed in conditions of privation – say, a stressed home without a lot of books – and enhanced in conditions of enrichment. These differences have a tragic cause: when children grow up in poor environments, they are unable to reach their full genetic potential. The lack of nurture holds back their nature.

You can see this relationship in the chart below. As socioeconomic status increases on the x-axis, the amount of variance in cognitive-test performance explained by genes nearly triples. Meanwhile, nurture generates diminishing returns. Although upper class parents tend to fret over the details of their parenting — Is it better to play the piano or the violin? Should I be a Tiger Mom or imitate those chill Parisian parents?— these details of enrichment become increasingly insignificant. Their children are ultimately held back by their genetics.

It’s a compelling theory, with significant empirical support. However, a number of studies have failed to replicate the Scarr-Rowe hypothesis, including a 2012 paper that looked at 8716 pairs of twins in the United Kingdom. This inconsistency has two possible explanations. The first is that the Scarr-Rowe hypothesis is false, a by-product of underpowered studies and publication bias. The second possibility, however, is that different societies might vary in how socioeconomic status interacts with genetics. In particular, places with a more generous social welfare system – and an educational system less stratified by income - might show less support for the Scarr-Rowe hypothesis, since their poor children are less likely to be cognitively limited by their environment.

These cross-country differences are the subject of a new meta-analysis in Psychological Science by Elliot Tucker-Drob and Timothy Bates. In total, the scientists looked at 14 studies drawn from nearly 25,000 pairs of twins and siblings, split rather evenly between the United States and other developed countries in Western Europe and Australia. The goal of their study was threefold: 1) measure the power of the Scarr-Rowe hypothesis in the United States 2) measure the power of the Scarr-Rowe hypothesis outside of the United States, in countries with stronger social-welfare systems and 3) compare these measurements.

The results should depress every American: we are the great bastion of socioeconomic inequality, the only rich country where many poor children grow up in conditions so stifling they fail to reach their full genetic potential. The economic numbers echo this inequality, showing how these differences in opportunity persist over time. Although America likes to celebrate its upward mobility, the income numbers suggest that such mobility is mostly a myth, with only 4 percent of people born into the bottom quintile moving into the top quintile as adults. As Michael Harrington wrote in 1962, “The real explanation of why the poor are where they are is that they made the mistake of being born to the wrong parents.” 

Life isn’t fair. Some children will be born into poor households. Some children will inherit genes that make it harder for them to succeed. Nevertheless, we have a duty to ensure that every child has a chance to learn what his or her brain is capable of. We should be ashamed that, in 21st century America, the effects of inequality are so pervasive that people on different ends of the socioeconomic spectrum have minds shaped by fundamentally different forces. Rich kids are shaped by the genes they have. Poor kids are shaped by the support they lack.

Tucker-Drob, Elliot and Bates, Timothy. “Large cross-national differences in gene x socioeconomic status interaction on intelligence,” Psychological Science. 2015.