Growing up in a society with a wide gap between the rich and the poor is linked to higher rates of depressive symptoms later in adulthood. This effect tends to be stronger for individuals with a genetic predisposition to depression, suggesting that an unequal economic environment can amplify biological risks. These findings were recently published in the journal Psychological Medicine.
Income inequality refers to how unevenly money is distributed across a population. High levels of this inequality have been consistently linked to negative health outcomes, including a higher risk of depression. Social scientists propose that environmental exposures during key developmental periods in childhood are especially important for future mental health.
An international team of scientists from institutions across the globe collaborated to explore how childhood economic environments interact with human genetics. They wanted to understand if early exposure to inequality could act as an adverse social condition that triggers genetic vulnerabilities.
“My colleagues and I are interested in how genetic and environmental influences work together to shape life trajectories,” said Margaret Gatz, a professor of psychology and senior scientist at the University of Southern California’s Center for Economic and Social Research. “This research is part of that larger effort.”
Gatz explained that the team wanted to determine the specific conditions under which genetic propensities play a larger or smaller role.
“The environmental conditions in this publication are levels of economic inequality in the society in which one grew up,” Gatz said. “Economic inequality has grown much larger in recent years in the United States. Also, there are marked differences in extent of economic inequality across different countries. We were particularly interested to understand what difference inequality made in people’s sense of their own well-being.”
A central concept in this research is gene-environment interaction. This occurs when a person’s physical surroundings or life circumstances change the way their genetic traits are expressed. The diathesis-stress model is a psychological theory suggesting that high-risk environments will have a greater negative impact on people who already have an underlying genetic risk for a specific condition.
Most genetic studies rely on self-reported stress, which can be subjective and biased. The authors decided to use the national income gap as an objective, external measure of environmental stress. They set out to test whether the wealth gap during childhood modifies the genetic risk for depression in midlife and older adulthood.
To explore these questions, the research team analyzed data from 69,924 participants. These individuals were part of the Interplay of Genes and Environment in Multiple Studies consortium. The sample included twins born between 1893 and 1979 from four countries: Australia, Denmark, Sweden, and the United States. Participants were between 22 and 103 years old when their depressive symptoms were assessed.
The scientists measured childhood income inequality by calculating the share of the national income held by the top one percent of earners in each country. They looked at the data for the years when each participant was between five and fifteen years old. To account for a country’s overall wealth, the authors also tracked the gross domestic product during those same years.
Gross domestic product is a standard measure of a country’s economic output and general standard of living. It helps researchers understand the basic resources available within a nation. By including both the inequality measure and the economic output measure, the researchers could separate the effects of wealth distribution from the effects of total national wealth.
The researchers also looked at genetic data. A subset of 6,256 participants had their DNA analyzed to calculate a polygenic index for major depressive disorder. A polygenic index is a score that estimates a person’s genetic likelihood of developing a certain trait or disease based on thousands of tiny genetic variations across their genome.
Because the sample included identical twins and fraternal twins, the researchers could use statistical models to estimate heritability. Identical twins share all of their genes, while fraternal twins share about half. Heritability measures how much of the variation in a trait is due to genetic differences within a population.
The data revealed that childhood exposure to higher income inequality was consistently associated with higher depressive symptom scores in adulthood. The researchers adjusted for factors like age, sex, and education. After these adjustments, they found that every one percent increase in the income share held by the top one percent of earners was linked to a moderate increase in depressive symptoms.
The authors found that this negative effect varied based on demographics. The link between inequality and depression was generally stronger for men than for women. The country’s overall wealth also changed the dynamic. The negative impact of inequality on depression was strongest in countries with either very low or very high economic output.
Education level also played a significant role in modifying the risks. The adverse effects of childhood inequality were most severe for those who completed less than a high school education. In contrast, the negative impact was greatly reduced and almost nonexistent for individuals who went on to obtain a college degree.
When examining the genetic data, the researchers found evidence of gene-environment interactions, particularly for men. Men who experienced high inequality during childhood and had a high genetic risk for depression reported the most severe depressive symptoms. This finding aligns with the diathesis-stress model, suggesting the harsh environment amplified their genetic risk.
The results looked slightly different for women. For women, both genetic risk and income inequality independently contributed to depression. The two factors did not show a compounding interaction effect like they did for men.
The twin models provided further evidence that the environment influences genetics. The authors discovered that the heritability of depressive symptoms changed based on the economic environment. In societies with high income inequality, genetic factors played a larger role in driving depression.
In more equal societies, the genetic predispositions for depression appeared to be suppressed or offset by the environment. The heritability of depressive symptoms was about 30 percent in areas with the lowest childhood inequality. This number jumped to nearly 37 percent in areas with the highest levels of inequality.
Summarizing the main findings, Gatz highlighted how early economic conditions relate to later mental health.
“Being exposed to greater economic inequality as a child was reflected in higher levels of depressive symptoms in adulthood,” Gatz said. “But additionally, in men, having a higher genetic predisposition for depression was associated with a notably stronger association between childhood exposure to inequality and adult depressive symptoms.”
Gatz noted that this compounding effect was distinct based on sex.
“In women, both genetic predisposition and exposure to inequality were important, but one did not enhance the effects of the other,” Gatz added. “And, overall in the population, genetic influences on whether individuals experienced symptoms of depression were stronger when there had been greater exposure to inequality. Conversely, with exposure to greater economic equality, genetic predispositions for depression tended to be suppressed.”
While these findings provide substantial insight, there are a few limitations to consider. Relying on the income share of the top one percent is a broad measure. It assumes that everyone in the country experiences the economic environment in exactly the same way, which is rarely the case.
Another limitation is the demographic makeup of the participants. The studies primarily included white participants from high-income, developed nations. This limits the ability to apply the findings to more diverse, global populations, especially those in less developed regions.
Readers should avoid interpreting the study to mean that economic inequality directly and independently causes depression. The research shows a correlation, but it cannot pinpoint the exact mechanisms at play. The researchers lacked the data to test if the depression was driven by specific community factors like personal poverty, financial strain, or a general sense of unfairness.
Gatz emphasized that the findings present a broad picture rather than a definitive predictor for any single individual.
“Although statistically significant, these effects are of course not the sole influence as to who experiences symptoms of depression,” Gatz said. “The results, however, illustrate the importance of considering both genetic and environmental influences in accounting for human health and well-being. Extent of societal economic inequality does matter to individual mental health.”
Moving forward, the research team plans to investigate how other systemic factors interact with human genetics.
“We continue to look at other aspects of inequality, including educational opportunities in a society,” Gatz said, “and how this interacts with reaching one’s full genetic potential with respect to intellectual capacities and accomplishments.”
The study, “The long reach of childhood income inequality: a multinational twin study of gene–environment interplay on adult depressive symptoms,” was authored by Andrew J. Petkus, Chandra A. Reynolds, Brian K. Finch, Kyla Thomas, Christopher R. Beam, Vibeke S. Catts, Malin Ericsson, Deborah G. Finkel, Carol E. Franz, William S. Kremen, Lisbeth Aagaard Larsen, Nicholas G. Martin, Matt McGue, Miriam A. Mosing, Jenae M. Neiderhiser, Marianne Nygaard, Nancy L. Pedersen, Anbupalam Thalamuthu, Keith E. Whitfield, Margaret Gatz, and the IGEMS Consortium.