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

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

  1. natselrox says:

    Good post!

    I had a few questions about the research and mailed Hanson to which he was kind enough to reply. There is an ongoing discussion (with Hanson) over here:

    If anyone’s interested, welcome! :)

  2. This is really great stuff–thank you for posting it. The links to previous materials are also excellent.

    I agree that “increasing human potential means supporting human development and reducing inequality.” And it is important to understand how social inequality has biological and brain consequences.

    However, I do worry about those who might use this research to “write off poverty as biological destiny.” Many people assume poverty and inequality cannot be addressed–in the U.S. and internationally–and could use such research to claim those with lower hippocampal grey matter density just cannot be helped.

    Do we know much about the plasticity or dynamic biology possibly involved? If the conditions of poverty change, will hippocampal grey matter density also improve?

    Thank you again!

  3. Maia Szalavitz says:

    Please be aware of the potential negative effects of this research on people who have grown up in poverty. Imagine what it’s like to read this if this is your personal background and you have a supposedly “toxic” brain.

    Also, please be aware that many people read this as “write them off, they’re doomed already, may as well send them to jail at 3,” rather than as” OMG we gotta do early childhood programs and decrease inequality.”

    Finally, when writing about this please also note that despite the very well documented horrific effects of poverty, chronic stress and inequality on the brain, THE MAJORITY WHO GROW UP POOR DO NOT DEVELOP ADDICTIONS, OTHER MENTAL ILLNESS OR COMMIT CRIMES.

    • Jamie says:

      I think Maia’s comments are dead on… this (and other) research on poverty should be interpreted with great care.

    • Jeffrey says:

      Please don’t take this question the wrong way. I’m not trying to be insensitive; this is a genuine question.

      Maria, you say that “despite the very well documented horrific effects of poverty, chronic stress and inequality on the brain, THE MAJORITY WHO GROW UP POOR DO NOT DEVELOP ADDICTIONS, OTHER MENTAL ILLNESS OR COMMIT CRIMES.”

      Assuming that’s true (I have no idea if it is or not, so I’ll trust you), what’s the real-world significance of a study like this? If the “horrific effects of poverty” on the brain do not translate into real-world consequences such as addictions, mental illness, and/or criminal activity, then is this (to take the original poster’s example) “like saying that slavery is both harmful to people and morally wrong because it impacts brains”?

      It just seems to me that either poverty-impacted brain development/function leads to real-world problems – thus lending at least some weight to the question of what really can be done to help those already living in poverty, or poverty-impacted brain development/function has little to no impact on real-world problems – thus implying that studies on the subject are purely academic exercises.

      Again, please don’t take this question the wrong way. I’m fully in support of finding ways to alleviate poverty – I’m just wondering whether immediately disqualifying the more uncomfortable implications of such research is the right thing to do.

  4. Maia Szalavitz says:

    P.S. I’ve been guilty of leaving those points out myself, which is why I’m trying to point it out now 😉

  5. Jamie says:

    I’m actually the first author of the paper and thought I would just thank you for the interesting (and accurate) distillation!

    Regarding hippocampal plasticity, I feel like there is lots of conflicting data & thinking about this. Many point to the effect of experience & the environment on the hippocampus (and reference the work of Bill Greenough & Bruce McEwen, both in rodents). This work underscores the hippocampus is greatly modified by experience (Greenough’s work in particular looked at environmental enrichment and its protective/beneficial effects on the hippocampus). However, there is additional data to suggest a “less plastic”/more genetically-heritable view of the hippocampus (a paper by Gilbertson in 2002 examining MZ-DZ human twins differentially exposed to trauma; a recent paper from a group at Wisconsin examining rhesus macaques and tracking brain activity).

    I think, in the end, the hippocampus may be heritable, but only in mid/high-SES environments. I hypothesize that a stressful environment could “trump” genes and cause major changes. Behavioral genetics work on the heritability of IQ found something similar to this propose idea, where IQ was very heritable in mid/high-SES environments. However, in low SES environments, the heritability of IQ greatly dropped off & the environment explained more of the variance (this is the work of Eric Turkheimer and colleagues, most notably).

  6. Maia Szalavitz says:

    Thanks, this whole area of research is really fascinating. I think Martha Farah has also had similar findings. Self promotion moment: I wrote about the ACE studies and the social modulation of the effects of trauma and poverty with Bruce Perry, MD, PhD. in Born for Love: Why Empathy Is Essential and Endangered.

    What’s really depressing is we know the importance of early childhood stuff like paid family leave, high quality daycare and preschool and prenatal care. We know how much it saves lives and prison and health costs and does good all around—and this research is showing the biology of it as well—but we don’t act on it and we continue to act as though we’re not all in it together and we’re not social beings and social connections and family life are just afterthoughts.

  7. Maia Szalavitz says:

    The real world significance is that a definite minority—up to 30%, varying with condition—do develop significant, sometimes disabling problems. So that’s why it matters and needs to be studied: the rates of all of these problems are way higher in the poor than in others.

    Addiction, for example, is *not* an equal opportunity disease. While drug use rates are pretty much the same across class, rates of addiction are higher in the poor (and this is not just because having a drug habit can make you run out of money). In the West, obesity, and everywhere heart disease, diabetes, stroke, etc. are all higher the lower you get on the socioeconomic scale.

    All of these things occur on a continuum, so even though the majority is resilient, that doesn’t mean we should ignore the rest. But what we do is focus relentlessly on pathology (ie, we don’t look much at the 85% of crack users who *don’t* become addicts) and it’s important to recognize resilience if we want to understand the problem fully.

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