Income inequality is growing in the U.S., and the problem is much worse than most people believe. For children, growing up poor hinders brain development and leads to poorer performance in schools, according to a study published this week in JAMA Pediatrics.

It has long been known that low socioeconomic status is linked to poorer performance in school, and recent research has linked poverty to smaller brain surface area. The current study bridges these converging lines of evidence by revealing that up to 20 percent of the achievement gap between high- and low-income children may be explained by differences in brain development.

Using a sample of 389 healthy children and adolescents from age four to 22, psychologist Seth Pollak and his colleagues at the University of Wisconsin–Madison compared scores on academic achievement tests with tissue volume in select areas of the brain. Researchers placed subjects in a magnetic resonance imaging machine to scan and measure gray matter volume in the temporal lobes, frontal lobes and hippocampus—brain areas that are critical to cognitive processes required for academic success and vulnerable to a person’s early environment. Some of the individuals came back for reassessment after 24 months and returned for follow-ups over a period of up to six years.

The researchers found that children who grew up in families below the federal poverty line had gray matter volumes 8 to 10 percent below normal development. They did not find differences between children from middle class and affluent families but those only 50 percent above the poverty line showed gray matter volumes 3 to 4 percent below the norm. In other words, more money does not necessary mean better outcomes but at a certain point a “drop-off” effect of income occurs where a lack of financial resources is detrimental to development. “The reason I think we’re not seeing a continuum is that humans are very hardy,” Pollak says, “I think the human child can learn to accommodate quite a wide variety of circumstances—what’s happening in extreme poverty is that we’re moving out of the range where the human brain can cope.”

Pollak was hoping to see the gap closing as children grew older, spent more time in school and outside their homes, but this was not the case. The gap in brain development was still present at age 22.

To solely address the effects of low income, the researchers excluded children who had other factors known to negatively affect brain development, such as a family history of psychiatric diagnosis or a risky pregnancy. The results were clear—the effects of low socioeconomic status are apparent even in kids who grew up otherwise healthy. “This was an important study,” says Joan Luby, a psychiatrist at the Washington University School of Medicine in Saint Louis who was not part of this study. It went one step further than the existing literature, she notes, and created a link between poverty and academic achievement. “It gives us a clear road map for future public health action.”

A recent report from the National Center for Education Statistics revealed that in 2013, 51 percent of students in U.S. public schools were from low-income families. Children who grow up in low-income families are exposed to more environmental stressors, such as less access to healthy food, unsafe neighborhoods and stressed parents. Pinpointing the specific causes will be difficult but in future studies Pollak and his group hope to identify how different social programs, such as free lunch programs or housing vouchers, can help children who grow up poor. “I used to think about poverty as a question of social policy. Now I think of it as a biomedical problem, an environmental condition or toxin that’s affecting children,” Pollak says.