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Growing up poor has long been linked to lower academic test scores. And there's now mounting evidence that it's partly because kids can suffer real physical consequences from low family incomes, including brains that are less equipped to learn.

An analysis of hundreds of magnetic resonance imaging (MRI) brain scans found that children from poor households had smaller amounts of gray matter in areas of the brain responsible for functions needed for learning, according to a new study published on Monday in JAMA Pediatrics. The anatomical difference could explain as much as 20 percent of the gap in test scores between kids growing up in poverty and their more affluent peers, according to the research.

The study adds to our understanding of the link between income and achievement. It’s well-established that, on average, poor children lag in school performance. But scientists are just beginning to unravel precisely how wealth affects learning.

Children in households below the federal poverty level—an annual income of about $24,000 for a family of four—had gray matter volumes 7 percent to 10 percent lower than what would be expected for normal development. About 20 percent of American children lived at this income level in 2013, according to Census data. Smaller gaps were evident for households considered "near poor," making up to 150 percent of the poverty level, currently about $36,000 for a family of four.

Kids living just above the "near poor" level looked statistically similar to children from much wealthier families.

“It was really when we started getting down into real poverty, real abject poverty, that we started seeing a difference,” says Seth Pollak, a professor of psychology at the University of Wisconsin-Madison and co-author of the study. The differences were evident in kids as young as 4, meaning they occur before kindergarten.

The research may understate the size of the effects. Pollak and his colleagues used data and brain images from an earlier National Institutes of Health study that tracked several hundred children and adolescents over several years. That study excluded people with learning disabilities, premature births, or family histories of psychiatric problems, because it was intended to measure a baseline for normal brain development. As a result, Pollak says, the sample included "the healthiest, most robust children living in poverty."

What accounts for the differences? Pollak suspects that poor children “are getting too little of things we need to develop the brain and too much of things that inhibit brain growth.” They may get less stimulation from parents, or lack things like crayons, children’s books, or games. Crowded environments or unstable homes may disrupt their sleep. Poor neighborhoods may not have grocery stores with fresh food, leading to nutritional deficits.

Pollak says the research has made him think of poverty as a medical problem, akin to exposure to lead paint, rather than strictly a social condition. That idea challenges a common American sentiment: “We like to believe in the United States that education is an equalizer, that everyone has a fair shot,” says Pollak. "This is sort of suggesting that we have some people entering kindergarten not getting a fair shot."

Other recent research has tied income to brain development. The journal Nature Neuroscience reported in March that brain surface area was linked to income, with the biggest effects noticeable among the poorest kids. "It’s only in the last few years that there’s been any systematic research asking about the biological side of the story," says John Gabrieli, a Massachusetts Institute of Technology neuroscientist who co-authored a separate article published in April about links between gray matter, income, and test scores. He wasn't involved in Pollak's research.

Gabrieli cautions that the differences in development don't mean poorer students can't catch up in the right circumstances. "We have so much very strong evidence that there’s lots of room for brain plasticity all the way through adulthood," he says.