Your education, job, income, neighborhood you live in: Together, these factors are considered to represent socio-economic status (SES) and contribute to a variety of health and social outcomes, from physical and mental health to academic achievement and cognitive abilities.
The brain acts as an obvious mediator between SES and many of these outcomes. But the mechanism by which it does this has remained unclear, and scientific studies have failed to show whether the impact of SES on the brain is encoded in our genes or dictated by the environment we live in.
In a new report in Scientists progress, an international research team led by scientists from the University of Pennsylvania and the Vrije Universiteit Amsterdam is working to distinguish the relative contributions of genes and environment. Using the largest dataset ever applied to this question, the team found evidence that genetic and environmental influences contribute to the impact of SES in a complex interplay with effects that span a variety of brain regions.
What we saw in the study is that part of the relationship between the brain and socio-economic status could be explained by genetics, but there is much more to this relationship that remains even after having consider genetics. This suggests that socio-economic conditions somehow get under the skin and may have additional negative influences on the social and economic disparities we see around us.”
Gideon Nave, professor of marketing, Penn’s Wharton School and co-author of the study
The work is the product of a large academic collaboration co-led by Philipp Koellinger de Nave and Vrije, a senior author of the study, called BIG BEAR, for Brain Imaging and Genetics in Behavioral Research. Martha Farah, another co-lead author of the work and a professor of psychology at Penn, is the lead researcher on the collaboration.
Mapping the imprint of SES in the brain
A large body of research has shown that SES has a signature in the brain.
“I study the relationship between SES and the brain,” says Farah, “and one question that always comes up is: what causes these differences? Are the characteristics of SES encoded in the genome, or is it life experience at different levels of SES have these differences We were able to show that it is both, and also that genes and environment seem to exert different effects on different parts of the brain.
In the work, the researchers used a massive dataset, the UK Biobank, to better understand these relative contributions. Previous studies either used smaller samples to investigate the link between the brain and SES or were inconsistent in defining SES. In contrast, the UK Biobank encompasses a wide range of data types, including brain scans and genomic sequencing as well as SES measurements, all collected in a standardized way. As a result, the research team was able to look for patterns among SSE factors and brain scan information for nearly 24,000 people.
Each individual was assigned two SES “scores”, one combining income, occupation and education level, and a second combining neighborhood and occupation. Looking at the two scores together, they accounted for about 1.6% of the change in total brain volume; a finding that had been seen previously.
The researchers then dug deeper into the brain scan data, looking for specific brain regions that tracked with SES. They found a whole host of different brain regions linked to SES, including a few surprises. It should be noted that the cerebellum, not analyzed by many previous studies, showed a substantial link with SES. Located near the brainstem, the cerebellum is responsible for movement and balance as well as higher-level functions involving cognition and learning.
“We see correlations emerging throughout the brain between SES and gray matter volume,” says Nave. “They are small, but with the large sample size of our study, we can be sure they are real.”
Adds Hyeokmoon Kweon, the first author of the study and a doctoral student at the Vrije Universiteit Amsterdam: “It is important to note that these small regional correlations do not imply that the overall relationship between the brain and SES is also weak. In fact, we can predict a considerable amount of SES differences by aggregating these small brain-SES relationships.
Nature versus nurture
Given that tens of thousands of individuals in the UK Biobank have also had their genomes sequenced, researchers could be looking for evidence of the genetic influence of SES in the brain. For this analysis, they created a unique index of SES and genetic linkages based on previous research that identified single nucleotide polymorphisms; one-letter variations of the DNA code; which are correlated with the SES.
Using this index, they found that genetics could explain just over half of the relationship between gray matter volume and SES in some regions. The prefrontal cortex and insula – responsible for abilities such as communication, decision-making and empathy – were found to be particularly strongly governed by genetic influence. However, the relationship between SES and gray matter volume in other brain regions – the cerebellum and lateral temporal lobe, for example – was less correlated with genetics, indicating that the alterations may instead be influenced by the environment.
Highlighting the influence that the environment can have, the researchers look at another variable in the data: body mass index (BMI). Although genetics plays a role in BMI, BMI also arises from non-genetic factors, including nutrition and physical activity. Even after controlling for known genetic links between brain anatomy and SES, they found that BMI may account for an average of 44% of the relationship between SES and gray matter volume.
The finding suggests that environmental factors, not just genetic determinants, that can contribute to high BMI, such as poor diet and insufficient exercise, can also manifest in brain structure.
A rationale for the intervention
The researchers say their findings, far from suggesting that there is nothing that can be done to improve the impact of SES on the brain, rather underscore that thoughtful policy-making could address health and social disparities linked to differences of SSE.
“The issue of genetic or environmental contributions to differences in SES is controversial, in part because of its perceived implications for policy,” Farah said. “Many people think that if the difficulties of people with low SES are caused by the environment, then you can and must modify the environment, but then you come to an illogical conclusion: insofar as they are genetic, it There is nothing you can do.Genetic-based problems can also be improved by environmental interventions, such as dietary changes for people with severe innate metabolic syndrome PKU or glasses for common vision problems.
Policy interventions could be a solution, the researchers say, addressing, for example, environmental justice concerns related to poorer neighborhoods. “If the air quality is worse in low-SES neighborhoods, it can trigger inflammation and other negative effects on the brain,” says Nave. “To cite just one example, regulations that mitigate air pollution could eliminate this damage and improve health and well-being at all levels, regardless of the neighborhood in which one lives. Free and high quality can do the same thing. Genetics, in this case, is not fate.”
Further studies are needed, according to the team, to move from identifying correlations to determining causes in terms of understanding the environmental effects of SES on the brain. “With more and more data available,” says Kweon, “I think we will soon be able to produce such studies, which will help shape targeted interventions.”