Stark and rising inequality plagues many countries, including the United States, and politicians, economists, and—fortunately—scientists, are debating its causes and solutions. But inequality’s effects may go beyond simple access to opportunity: a new study finds that family differences in income and education are directly correlated with brain size in developing children and adolescents. The findings could have important policy implications and provide new arguments for early antipoverty interventions, researchers say.
Researchers have long known that children from families with higher socioeconomic status do better on a number of cognitive measures, including IQ scores, reading and language batteries, and tests of so-called executive function—the ability to focus attention on a task. More recently, some studies have found that key brain areas in children of higher socioeconomic status—such as those involved in memory or language—tend to be either larger in volume, more developed, or both. However, these studies have suffered from some important limitations: For one thing, they don’t adequately distinguish socioeconomic status from racial background, which in the United States are difficult to tease apart because nonwhite groups tend to have higher poverty levels. And few studies treat family income and education levels as independent factors, even though they can act differently on the child’s developing brain. For example, income may be a better indicator of the material resources (such as healthy food and medical care) available to a child, whereas more highly educated parents may be better able to stimulate their child’s intellectual development.
To get around some of these limitations, a research team scanned the brains of 1099 children and young adults, ranging from 3 to 20 years old, using MRI. The researchers, led by Kimberly Noble of Columbia University and Elizabeth Sowell of Children’s Hospital Los Angeles in California, both cognitive neuroscientists specializing in child development, recruited subjects in collaboration with researchers at nine U.S. universities and hospitals, using Internet and community advertising as well as word of mouth.
The MRI scans allowed the team to measure the surface area of the subjects’ cerebral cortices, the outer layer of the brain where most advanced cognitive processing takes place, including language, reading, and executive functions. The researchers chose to measure cortical surface area because previous research had shown that it increases throughout childhood and adolescence as the brain develops, thus making it a potentially sensitive indicator of intellectual abilities. Studies in both animals and humans have suggested that the cortex can grow larger as a result of life experiences, although genetic factors may partly influence its overall size. The team also administered a battery of standard cognitive tests to the subjects and took DNA samples to control for the factors of race and genetic ancestry.
The results, published online this week in Nature Neuroscience, showed that cortical surface area was indeed correlated with different measures of socioeconomic status. Parental education—the number of years that parents had gone to school—showed a linear correlation with overall cortical surface area, especially for regions of the brain involved in language, reading, and executive functions. As a rough approximation, the children of parents with only a high school education (12 years of education or less) had 3% less cortical surface area than children whose parents had attended universities (15 years or more), Noble and Sowell told Science.
The team also found a significant correlation between cortical surface area and family income levels, which ranged from less than $5000 per year to more than $300,000. This was not a linear correlation, however. Instead, at the very lowest income levels, each incremental increase in income led to relatively greater increases in cortical surface area, whereas the influence of income tended to level off at higher levels. Nevertheless, Noble and Sowell say, the difference between lower and higher incomes is dramatic: Children from families making $25,000 per year or less have cortical surface areas roughly 6% smaller than those making more than $150,000.
The team also found that cortical surface area was related to performance on at least some cognitive tests, especially those measuring executive functions and memory. Finally, race and ethnicity had no effect on any of these correlations. “The links between socioeconomic status and brain structure were the same across individuals, regardless of racial background,” Noble says.
In their paper, the team cautions that despite these clear correlations between socioeconomic status and the size of the cerebral cortex, the reasons for the correlations are not yet clear. Low socioeconomic status could inhibit brain growth due to family stress, greater exposure to environmental toxins, or insufficient nutrition, while higher status families might be able to provide more “cognitive stimulation” to their children. Nevertheless, the researchers point to the particularly low cortical surface areas of low-income children—and the differences that even small, incremental increases in income can make—as evidence that antipoverty measures could make a big difference in both brain size and intellectual achievement. “The implications for public policy are substantial,” Sowell says. “The brain develops over a very long period, throughout childhood and adolescence,” she adds, suggesting that enriching the environment of a child “at any point in development” can make a big difference in his or her abilities.
But unknown genetic factors that influence brain size and also correlate with income could play a role in the results, says Ian Deary, a psychologist at the University of Edinburgh in the United Kingdom who is well known for his work on intelligence. He cites recent studies concluding that both genetic and environmental factors influence socioeconomic status.
Still, Martha Farah, a cognitive neuroscientist at the University of Pennsylvania, says that the study is “a real advance in characterizing how brain development differs” between children of lower and higher socioeconomic status, calling it a “crucial first step” in understanding how income and education levels “shape human development.” She agrees that the study provides compelling support for the idea of alleviating childhood poverty. “Even without neuroscience, the case for investment in society’s poor children is very strong,” she says. “But if brain imaging helps to focus people’s attention on the problem of childhood poverty, that’s great.”