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Where children live and what their household’s socioeconomic status is leaves a mark on their brains, a new study in Science finds. The results suggest that the fewer opportunities a child’s zip code affords, the more tired and stressed their brain appears—and that socioeconomics by far outweighs hundreds of other possible environmental factors in determining a child’s brain function and structure.

“Socioeconomic came out ahead by like a million miles,” says Nico Dosenbach, the study’s senior author and a professor of neurology at the Washington University in St. Louis. Other factors generally thought to be important to child brain development, such as a child’s culture and overall health and their caregivers’ parenting style, didn’t rise above the fold at all, he adds.

Dosenbach and his colleagues used a dataset from the Adolescent Brain Cognitive Development (ABCD) Study, a long-term investigation of brain development and child health in the U.S. Using thousands of children’s brain scans, the scientists made maps of each child’s brain function and structure and then weighed them against 649 variables.

These variables included IQ and cognitive test measures, demographic and cultural information, and mental and physical health records, as well as the child’s score on a measure called the Child Opportunity Index (COI). The COI measures and maps the quality of resources such as safe housing, food access, and schools nearby. Then the researchers looked for patterns in the brain maps that could reveal which factors were most associated with significant changes in the brain. The researchers compared what they were seeing with another, totally unrelated adult sample—the U.K. Biobank—and found that the same patterns persisted.

“A lower socioeconomic brain—so a child who grows up at the lower end, their brain looks more tired and stressed out,” Dosenbach says. “It doesn’t look dumber. The pattern of association completely spares the cognition areas of the brain.”

That’s important, Dosenbach stresses: past research has suggested that socioeconomic status is linked to IQ and cognitive scores, but the new results indicate the reason may be entirely to do with how sleep-deprived and stressed a child is when they are tested—not their basic cognitive ability. That finding came as a shock to Dosenbach and the study’s first author, Scott Marek.

“At the very least, I thought there would still be something there,” says Marek, an assistant professor of radiology at Washington University School of Medicine. Instead, the researchers found that any associations between cognition and the brain disappeared after they adjusted for socioeconomic status. “Place matters a lot for pretty much everything in our lives, so why not the brain as well?” Marek says. “It literally is the factor that permeates all brain behavior association studies.”

Dosenbach says the results have challenged his assumptions about standardized testing for kids—and about screen time. One of the less prominent but still significant variables the team found to have some effect on brain function was how much screen time a child got. He points to journalist and psychologist Jonathan Haidt’s book The Anxious Generation, which argues that screen time is causing a mental health epidemic in kids. Dosenbach didn’t buy Haidt’s argument because it was based on correlations, he says, but he feels unable to argue with his data. “My daughter was about to get her [first] real cell phone, and I pulled the plug on it,” he says.

More broadly, the findings underscore the importance of early childhood circumstances on the developing brain. They do not, however, suggest that a child’s zip code determines their destiny. Marek says he hopes his team and other researchers can work toward developing interventions to combat sleeplessness and stress. Importantly, most of the changes associated with socioeconomic status that the researchers found were in brain function, as opposed to structure, indicating they may not last if those pressures are addressed.

In an accompanying article also published in Science, University of Pennsylvania neuroscientists Lucinda Sisk and Theodore Satterthwaite write that the findings “highlight the need for societal-level policies that provide early support for families.”

The study does have some key limitations, Marek says. For one, it isn’t clear how early in life children’s environment begins to weigh on their brains. The study also included ABCD data from just two time points in children’s lives, so it’s unknown whether the changes the team saw stick around through their teenage years or if they change with age.

The study also doesn’t account for the children’s genetics, such as by calculating and factoring in their polygenetic risk scores, says Torkel Klingberg, a professor of neuroscience at the Karolinska Institute in Sweden, who was not involved in the new study. These are measures that estimate a person’s genetic predisposition to a disease or a trait, such as educational attainment.

“Environment is super important for the brain development, for cognitive function, but so is genetics,” he says. “In order to draw firm conclusions, you really need to consider the effect of genetics.”

Marek says that at the time they did the analysis, the ABCD dataset didn’t include that information. The study does account for genetic ancestry, and it finds no correlation between that and a child’s brain.

“The story is fundamentally about place, right? It’s not race; it’s fundamentally about where you live. Doesn’t matter what color your skin is, what your family history is. The zip code is the thing that matters,” he says. At the same time, Marek and Dosenbach argue that trying to make sure kids are getting enough sleep and are less stressed in their immediate environment are relatively achievable and inexpensive interventions—and could have significant impacts on brain development.

“America as a country is extraordinarily rich,” Marek says, “and I think the hopeful message here is that, yeah, a lot of these effects seem like they are reversible, and they’re not set in stone.”

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