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 recently 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.