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 study discovered 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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With constant, excessive stress, the hippocampus becomes overloaded. Eventually, the structure itself gets damaged, which can then impair memory and learning. This stress-atrophy effect is most pronounced in people with PTSD, whose hippocampi tend to have smaller volumes and less activation during memory tasks. But it's also apparent in low SES people, who unsurprisingly have higher levels of cortisol.

It gets worse: Our brains' fight-or-flight response typically subdues after a threat is resolved. "Once you realize the threat isn't there or it's over, you want to quickly bring down the stress response, because sustained exposure to stress hormones is bad for you," Farah says. But some research has found that the amygdala, a region next to the hippocampus that's responsible for quickly perceiving threats and forming emotional memories, is more active in low SES people. Show a poor person—or even someone who was once poor—and a rich person pictures of threatening faces, and the poor person's amygdala will likely become more activated for longer. In other words, in response to the same stressor, people of low SES may get more stressed than people of high SES.

This makes sense: If you have lots of threats in your environment, like in a bad neighborhood, you'll learn to become hyper-vigilant. The problem is that, in turn, you may become more sensitive to the deleterious effects of stress. The consequence is a cycle of constant anxiety and altered brain function.

More acutely, the mental and emotional overload of poverty can impair self-control, which is critical to academic, professional, and life success. Through a neuroscience lens, it's understandable that higher SES in adults has been linked to resisting impulses and delaying gratification. Conversely, one study sums, the poor may be likelier than others "to behave in ways that can perpetuate a disadvantaged state."

So here's what all this means: Someone born into poverty already has the bad luck of being poor. Most likely, she's had bad access to healthcare, an inadequate education, unsafe places to play, a poor diet, high exposure to pollution and toxins, and parents who didn't have time to read to her at night. As if that's not bad enough, growing up in poverty then changes certain regions of her brain, which may place her at a further disadvantage in modern society.

"Yes, it is grim," Farah says, but her research makes her optimistic. By studying the effects of early socioeconomic deprivation on brain development, she and others hope to gain insight into how to prevent or reverse the effects of low SES. And though she admits that neuroscience hasn't yet produced concrete help for the poor, the science is young; the earliest studies on poverty in the brain are just a decade old. The more we learn, the better we can design interventions to outsmart poverty and protect our children's brains.

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