Does Lack of Income Take Away the Brain’s Horses?

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And I don’t mean the pretty horses people ride, but the hippocampus (or sea horse) circuits in your brain, which are crucial to memory. New research in PLoS One, Association between Income and the Hippocampus, demonstrates a link between lower socioeconomic status and lower hippocampal grey matter density.

In Wednesday’s round-up I linked to Philip Cohen’s post, Income gradient for children’s mental health. Here’s the opening graph so you can get a sense of the gravity of the situation. The percentage of children with serious mental or behavioral difficulties is shown as a percentage on the left. The drop-off as income rises is dramatic.

In 2008 we documented that poverty poisons the brain:

As the article explained, neuroscientists have found that “many children growing up in very poor families with low social status experience unhealthy levels of stress hormones, which impair their neural development.” The effect is to impair language development and memory — and hence the ability to escape poverty — for the rest of the child’s life. So now we have another, even more compelling reason to be ashamed about America’s record of failing to fight poverty.

And then in 2009, we focused on how it’s really the social side of things doing the poisoning:

Empirical research on the connection between poverty and intellectual development can cut both ways—leading some to write off poverty as biological destiny, and others to look deeper into missed opportunities to lift youth over economic barriers…

While I advocate for the role that brain processes can play in social theory, the sword cuts both ways. Referencing the brain as central mediator of poverty hides the truth, and distorts our understanding. To take a more extreme example to illustrate the same point, it’s like saying that slavery is both harmful to people and morally wrong because it impacts brains.

This new research brings us back to a focus on the brain. The article, whose lead author Jamie Hanson is a graduate student in psychology at Wisconsin-Madison, brings a broader focus than just stress, through cortisol, acting as poison to the developing brain.

Facets of the post-natal environment including the type and complexity of environmental stimuli, the quality of parenting behaviors, and the amount and type of stress experienced by a child affects brain and behavioral functioning. Poverty is a type of pervasive experience that is likely to influence biobehavioral processes because children developing in such environments often encounter high levels of stress and reduced environmental stimulation.

This study explores the association between socioeconomic status and the hippocampus, a brain region involved in learning and memory that is known to be affected by stress. We employ a voxel-based morphometry analytic framework with region of interest drawing for structural brain images acquired from participants across the socioeconomic spectrum (n = 317). Children from lower income backgrounds had lower hippocampal gray matter density, a measure of volume. This finding is discussed in terms of disparities in education and health that are observed across the socioeconomic spectrum.

Here is the summary of their conclusions at the start of the discussion:

This study was designed to examine the possible association between household family income and the hippocampus, a brain region central to many important cognitive and emotional processes. We identified an association with the hippocampus and income, as hypothesized. The hippocampus has previously been found to be associated with quality of environmental input and stress. Taken together, these findings suggest that differences in the hippocampus, perhaps due to stress tied to growing up in poverty, might partially explain differences in long-tern memory, learning, control of neuroendocrine functions, and modulation of emotional behavior.

And one graph so you can get a sense of the data, this one a scatterplot that shows the association between left hippocampal gray matter and income.

This research was done in the US. Given how Hadley and Patil have shown a link between food insecurity and mental health, and Panter Brick and colleagues have shown how inequality, mediated through trauma, links to mental distress, the fact that poverty poisons the brain around the world should be a strong working hypothesis. That means that increasing human potential means supporting human development and reducing inequality.

As I showed last fall, experience gets under the skin, particularly things like poverty, inequality, and stress, as well as social relationships.

Hertzman and Boyce outline one major study they have undertaken in British Columbia on early child development, including physical, social, emotional, language/cognitive and communication domains, and measured through the early development instrument (EDI), given to kindergarten teachers to assess children in these different domains. These rankings are then linked to neighborhood socioeconomic characteristics. What is striking, especially given how social causation is described, is that these neighborhood characteristics have general explanatory power, even if in specific domains the linkage can be less.

More than 40% of the variance for vulnerability on one or more scales can be explained by neighborhood socioeconomic characteristics, which clearly demonstrates the strength of the emerging gradient in basic developmental competencies (334).

The links between poverty, inequality, stress, and brain development are no longer ideal speculation. This is robust research, even if political powers want to either ignore it or favor strategies aimed at the middle class to get votes and support early schooling as a stop-gap band-aid against the larger reality.

The brain shows it. If only we could see what’s all around us, beyond our own skin, our own children, our own neighborhood.

Link to PLoS One article by Jamie Hanson et al., Association between Income and the Hippocampus

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