New research that shows poor children have smaller brains than affluent children has deepened the national debate about ways to narrow the achievement gap.

Neuroscientists who studied the brain scans of nearly 1,100 children and young adults nationwide from ages 3 to 20 found that the surface area of the cerebral cortex was linked to family income. They discovered that the brains of children in families that earned less than $25,000 a year had surface areas 6 percent smaller than those whose families earned $150,000 or more. The poor children also scored lower on average on a battery of cognitive tests.

The region of the brain in question handles language, memory, spatial skills and reasoning, all important to success in school and beyond.

The study, published last month in Nature Neuroscience, is the largest of its kind to date. It was led by Kimberly Noble, who teaches at both Columbia University's Teachers College and the university's medical school. Elizabeth Sowell, of Children's Hospital Los Angeles, was the senior author.

"We've known for so long that poverty and lack of access to resources to enrich the developmental environment are related to poor school performance, poor test scores and fewer educational opportunities," Sowell said. "But now we can really tie it to a physical thing in the brain. We realized that this is a big deal."

The study is part of a new and growing body of research on children's brain structures that has been made possible by recent technological advances in magnetic resonance imaging.

"It's only been in the past 20 years that we could have done this with living, developing children," said Sowell, who published a pioneering 1999 study that found the brain is still developing past adolescence, contrary to earlier beliefs that brain growth was complete by the teen years.

The research comes at a time when a majority of U.S. public school students come from low-income families and the academic achievement gap between poor and more-affluent children is growing. Policymakers are increasingly concerned about ways to reduce the gap, which is apparent as early as kindergarten.

In another study that has been accepted for publication in Psychological Science, a team led by neuroscientist John Gabrieli of MIT found differences in the brain's cortical thickness between low-income and higher-income teenagers. The study linked that difference for the first time to standardized test scores: 57 percent of the poor children scored proficient in math and reading tests given annually in Massachusetts, compared with 91 percent of the higher-income students.

"The thing that really stands out is how powerful the economic influences are on something as fundamental as brain structure," Gabrieli said. "It's just very striking."

The new research does not explain possible reasons for the brain differences. And that has created some concern that the findings will harden stereotypes and give an impression that children who are born into poverty lack the physical capacity to succeed academically.

"Some people feel if you show these brain differences, you're politically condemning the poor," Gabrieli said. "Which is the opposite, I think, of what we need to do. I think we want to understand adversity and minimize adversity."

Noble and Sowell have two theories about why poor children have smaller brains. One is that poor families lack access to material goods that aid healthy development, like good nutrition and higher-quality health care. The other theory is that poor families tend to live more chaotic lives, and that stress could inhibit healthy brain development.

Noble has embarked on a new study to try to answer that question. She has begun a pilot study to investigate whether giving low-income mothers a small or large monthly sum of cash impacts the cognitive development of their children in the first three years of life. She plans to recruit 1,000 low-income mothers from around the country, half of whom would receive $333 a month, while the other half would receive $20 a month for three years. That research is expected to take five years.

But James Thompson, a psychologist at University College London, has a third theory.

"People who have less ability and marry people with less ability have children who, on balance, on average, have less ability," he said. Thompson noted that there is a genetic component to intelligence which Noble and Sowell failed to consider.

"It makes my jaw drop that we've known for years intelligence is inheritable and scientists are beginning to track down exactly how it happens," Thompson said. "The well-known genetic hypothesis has not even had a chance to enter the door in this discussion."

Charles Murray, a conservative political scientist who argues there is a relationship between intelligence and economic class in his book "The Bell Curve," said genetics cannot be ignored.

"It is confidently known that brain size is correlated with IQ, IQ measured in childhood is correlated with income as an adult, and parental IQ is correlated with children's IQ," Murray wrote in an e-mail. "I would be astonished if children's brain size were NOT correlated with parental income. How could it be otherwise?"

In releasing their study, Noble and Sowell emphasized that the brain can grow and change. "That is a very critical point," Noble said. "The brain is incredibly able to be molded by experience, especially in childhood."

Mike Feinberg, a co-founder of the Knowledge is Power Program (KIPP), a network of 162 charter schools in 20 states and the District of Columbia that educates 59,000 students, 85 percent of whom have low family income, said all children are capable of learning regardless of their backgrounds or economic situation.

"We've been in the business of growing brains for 20 years now," Feinberg said. "For the vast majority of children, there is nothing physically about them that sets them up for success or failure as they start school. There are certainly societal circumstances that make it easier or harder for that child to learn on any given day. And certainly, children in poverty are going to develop more physical issues as well if they're not taken care of. But are they able to learn? Absolutely."

The Obama administration has increasingly promoted the idea that the country should provide early childhood education for low-income 3- and 4-year-olds to give them a boost before they get to kindergarten. Last week, Education Secretary Arne Duncan said if he had one more federal dollar to spend on education, he would funnel it to early childhood.