Growing up in a society with large disparities between rich and poor is associated with higher rates of depressive symptoms in adulthood and beyond. This effect tends to be stronger in people with a genetic predisposition to depression, suggesting that unequal economic environments may amplify biological risk. These findings were recently published in the journal psychiatry.
Income inequality refers to the degree to which money is unevenly distributed across the population. High levels of this inequality are consistently associated with negative health outcomes, including increased risk of depression. Social scientists have proposed that environmental exposure during critical developmental periods in childhood is particularly important for future mental health.
An international team of scientists from institutions around the world collaborated to investigate how early childhood economic environment interacts with human genetics. They wanted to understand whether early exposure to inequality can act as an adverse social context that causes genetic vulnerability.
“My colleagues and I are interested in how genetic and environmental influences work together to shape life trajectories,” said Margaret Gatz, professor of psychology and senior fellow at the Center for Economic and Social Research at the University of Southern California. “This study is part of that larger effort.”
Gatz explained that the research team wants to determine the specific conditions in which genetic predispositions play a greater or lesser role.
“The environmental conditions in this publication are the level of economic inequality in the society in which a person grew up,” Gatz said. “Economic inequality has increased significantly in the United States in recent years, and there are notable differences in the degree of economic inequality across countries. We were particularly interested in understanding how inequality makes a difference in people’s sense of well-being.”
A central concept in this study is the interaction of genes and environment. This occurs when a person’s physical or living environment changes the way their genetic traits are expressed. The diathesis-stress model is a psychological theory that suggests that high-risk environments have a greater negative impact on people who already have an underlying genetic risk for certain diseases.
Most genetic studies rely on self-reported stress, which can be subjective and biased. The authors chose to use national income inequality as an objective external measure of environmental stress. They set out to test whether wealth disparities in childhood alter genetic risk for depression in midlife and old age.
To explore these questions, the research team analyzed data from 69,924 participants. These people were part of a gene-environment interaction consortium in multiple studies. The sample included twins born between 1893 and 1979 in four countries: Australia, Denmark, Sweden, and the United States. Participants’ ages at the time of depressive symptom assessment ranged from 22 to 103 years.
Scientists measured childhood income inequality by calculating the share of each country’s top 1 percent of earners in its national income. They looked at data from years when each participant was between 5 and 15 years old. To account for a country’s overall wealth, the authors also tracked gross domestic product for the same year.
Gross domestic product is the standard measure of a country’s economic output and general standard of living. It helps researchers understand the basic resources available in the country. By including both a measure of inequality and a measure of economic output, the researchers were able to 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 particular trait or disease, based on thousands of small genetic variations across the genome.
Because the sample included identical and dizygotic twins, researchers were able to use statistical models to estimate heritability. Identical twins share all of their genes, while fraternal twins share about half of their genes. Heritability measures how much variation in a trait is due to genetic differences within a population.
The data revealed that exposure to high income inequality in childhood was consistently associated with higher scores of depressive symptoms in adulthood. The researchers controlled for factors such as age, gender, and education. After these adjustments, we found that each 1 percentage point increase in the share of income held by the top 1 percent of earners was associated with a modest increase in depressive symptoms.
The authors found that this negative effect varied based on demographics. The association between inequality and depression was generally stronger for men than for women. National wealth as a whole also changed power relations. The negative effect of inequality on depression was strongest in countries with very low or very high economic output.
Educational level also played an important role in varying risk. The negative effects of childhood inequality were most severe for those with less than a high school education. In contrast, for individuals with a college degree, the negative effects were significantly reduced to almost nonexistent.
When researchers looked at genetic data, they found evidence of gene-environment interactions, especially for men. Men who experienced greater inequality in childhood and were at higher genetic risk for depression reported the most severe depressive symptoms. This finding is consistent with the predisposition-stress model and suggests that the harsh environment amplified genetic risk.
For women, the results appeared to be slightly different. For women, genetic risk and income inequality both independently contribute to depression. The two factors did not show a combined interaction effect as in the case of men.
The twin model provided further evidence that environment influences genetics. The authors found that the heritability of depressive symptoms varied based on economic environment. In societies with high income inequality, genetic factors play a greater role in causing depression.
In more egalitarian societies, genetic predisposition to depression appears to be suppressed or offset by the environment. The heritability of depressive symptoms was about 30 percent in regions with the lowest childhood inequality. In regions with the highest levels of inequality, this figure jumped to nearly 37%.
Summarizing the main findings, Gatz highlighted how early economic circumstances relate to later mental health.
“Exposure to greater economic inequality in childhood was reflected in higher levels of depressive symptoms in adulthood,” Gatz said. “But in addition, a higher genetic predisposition to depression in men was associated with a significantly stronger association between unequal exposure in childhood and adult depressive symptoms.”
Gatz noted that this combined effect differs by gender.
“For women, both genetic predisposition and exposure to inequality were important, but the effects of one did not strengthen the effects of the other,” Gatz added. “And for the population as a whole, when exposure to inequality was greater, the genetic influence on whether an individual experienced symptoms of depression was stronger. Conversely, when economic equality was greater, genetic predisposition to depression tended to be suppressed.”
Although these findings provide important insights, there are several limitations that should be considered. Relying on the income share of the top 1 percent is a broad measure. It assumes that all citizens experience the economic environment in exactly the same way, which is rarely the case.
Another limitation is the demographic composition of the participants. The study included primarily white participants from high-income developed countries. This limits the ability to apply research findings to more diverse global populations, especially those in less developed regions.
Readers should avoid interpreting the findings that economic inequality directly and independently causes depression. Although research shows a correlation, it is not possible to pinpoint the exact mechanism that is actually occurring. Researchers lacked data to test whether depression was caused by specific community factors, such as personal poverty, economic strain, or a general sense of inequity.
Gatz emphasized that the findings are not definitive predictions for specific individuals, but rather represent the overall picture.
“Although statistically significant, these effects are, of course, not the only effects on experiencing symptoms of depression,” Gatz said. “However, this result shows the importance of considering both genetic and environmental influences when considering human health and well-being. The degree of social and economic inequality influences an individual’s mental health.”
The research team will now investigate how other systemic factors interact with human genetics.
“We continue to look at other aspects of inequality, such as educational opportunities in society,” Gatz said, “and how this interacts with maximizing genetic potential in terms of intellectual ability and achievement.”
The study, “Long-Term Effects of Childhood Income Inequality: A Multinational Twin Study of Gene-Environmental Interactions on Adult Depressive Symptoms,” was authored by Andrew J. Pecas, Chandra A. Reynolds, Brian K. Finch, Kyla Thomas, Christopher R. Beam, Vibeke S. Katz, Marin Eriksson, Deborah G. Finkel, Carol E. Franz, William S. Klemen, and 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.
