The stress of poverty can change the brain in ways that further disadvantage the poor.
We already know the poor are getting poorer. The proportion of American adults living in low income families increased from 25 percent in 1971 to 29 percent in 2011. And growing up poor ups the chances that you’ll also be poor as an adult. But neuroscientists are beginning to see this trend on a new level as they study the impacts of low socioeconomic status on childhood brain development.
Scientists have long researched how income, wealth, prestige, and education—socioeconomic status, or SES—relate to other outcomes. They’ve consistently found higher SES individuals outperform lower SES individuals on intelligence tests and school achievement. One studydiscovered that the average IQ of a group of children from poor inner city mothers was just 80 (the average is 100 for every age). As parental income declines, so do their children’s reading and memory abilities. One study found that those who spend their entire childhood in poverty score 20 percent lower on working memory tests than children who have never been poor. Language abilities also correlate with SES. One classic study demonstrated that three-year-olds from professional families had two-times larger vocabularies than children whose families were on welfare.
“So we shouldn’t be surprised to see SES reflected in people’s brains,” says Martha Farah, a cognitive neuroscientist at the University of Pennsylvania. Indeed, recent MRI studies reveal widespread differences in children’s brain structure depending on their socioeconomic status, including size differences of brain areas used for memory and learning, executive function, and others. Some of these differences, like cortex size, are observable as early as one month of age.
In one study, carried out by Kim Noble of Columbia University, children from families making $25,000 a year or less had six percent less cortical surface area—the outer layer of the brain associated with language, reading, and executive functions that continues to grow and develop into adolescence—than children from families earning more than $150,000. The effects of income on brain structure were especially pronounced in the poorest children.
Critically, socioeconomic status is at least partially the cause—rather than the effect—of these differences. The earlier in life you experience poverty, the lower your cognitive achievement is likely to be. The longer you spend in poverty, the worse your working memory gets. Perhaps most compellingly, research suggests that nearly half of IQ differences in adopted children are due to the SES of their adoptive family rather than, say, genetics. Even controlling for potentially confounding factors like ancestry and health, research shows clear brain differences between low and high SES children.
Such differences may ultimately perpetuate poverty. The stress of poverty can change the brain in ways that further disadvantage the poor in modern society.
For example, stressful environments can impair a child’s developing executive function—the skills that help us think, learn, plan, focus, develop strong vocabularies, synthesize abstract concepts, and succeed in school. In fact, home environment during one’s preschool years is a better predictor of first grade executive function than either child care or school classroom quality. Likewise, research suggests that stress alone explains the association between poverty and poor working memory.
One explanation for why stress so significantly affects brain function centers around how chronic stress impacts the hippocampus, a brain region that’s central to forming memories, learning conceptual information, and regulating the stress response. When we’re stressed, the stress hormone cortisol floods the hippocampus and other areas of the brain, like the prefrontal cortex.
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