Low Income is consistently related to brain structure and mental health
Kids growing up in lower income households tend to have more mental health problems, like depression and anxiety, than kids from higher income households, and many studies have also found poverty to be associated with structural differences in the developing brain. One of the most common findings is that children from low income households have a smaller hippocampus than children from higher income households. The hippocampus is central to memory and learning, and it is also uniquely sensitive to chronic stress. Animal studies show that consistently elevated stress hormones, such as cortisol, can reduce the formation of new synapses (or connections between neurons) in this region of the brain. We investigated the associations between family income and hippocampal volume and between family income and mental health in the Adolescent Brain and Cognitive Development (ABCD) study of over 11,000 9-11 year old youth from across the country. Like previous studies have found, low family income was associated with smaller hippocampal volume and more mental health problems.
Differences between US states
Because the ABCD study collected data from 21 sites in 17 states across the country, we also had a unique opportunity that has not been possible with previous studies on this topic, which typically are conducted at a single place and time. We were able to look at if characteristics of those places influenced the nature of the associations between family income and brain structure and mental health. We found that while, overall, children growing up in poverty had smaller hippocampal volume, how much smaller it was depended on the state that they lived in. We wanted to know why this might be. We thought that the 2 most likely explanations were the cost of living in those states - how expensive they are - and the strength of that state’s social safety net. That turned out to be the case. When youth lived in states that were more expensive, their hippocampal volume was even smaller, but this effect was counteracted if those states also had a stronger social safety net. We see a really similar pattern for depression and anxiety (i.e. internalizing problems). If we look just within those more expensive states, the magnitude of disparities in brain structure is reduced by about a third, while mental health disparities are cut almost in half by having a strong social safety net.
Measuring the social safety net
The specific programs we looked at were the Earned Income Tax Credit (EITC), Temporary Assistance for Needy Families (TANF; commonly referred to as "welfare"), and Medicaid expansion under the Affordable Care Act. To quantify the strength of the social safety net, we calculated the average monthly dollar amount of cash assistance that a poor family might expect to get from either the EITC or TANF and coded whether or not they had expanded Medicaid at the time the data was collected. We consider the measures to be proxies for the overall strength of the social safety net in that region, and therefore do not consider these analyses to be evaluations of specific policies. There are also other differences between these states that could explain these findings, but we tried to rule as many of those out as we could by conducting supplemental analyses controlling for a wide array of state characteristics, from the racial and ethnic makeup of the sample within that state to its population density to measures of education equity in those states. The results held consistently across these analyses.
Implications for public policy
Many studies have shown that the impacts of poverty are observable in children’s brains. While correlational, the results of our recent study suggest that policy decisions on things like Medicaid expansion and the generosity of cash assistance for families in poverty matter for the brain development and mental health of children. We therefore think that policymakers should take these findings into consideration as they weigh policy decisions like expanding Medicaid in their state or reviving the extended child tax credit, which cut child poverty almost in half.
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