Researchers have, for decades, tried to catalogue racial disparities and identify what causes them. The first part of that task turns out to be the easy part. National records of heart disease, cancer, stroke and diabetes all tell the same grim story about a disproportionate harm to Black people in the United States. But only part of those statistics can be directly traced to greater levels of poverty and instances of medical bias.

Increasingly, researchers have been exploring how the experience of racism itself might affect the body. Being on the receiving end of unfair or discriminatory behavior every day, they suggest, is a kind of chronic stress that can wear down defenses to the point that it promotes disease. One major building block of such research came in 1997, with the development of the Everyday Discrimination Scale—a self-reported measure of how much racism a person experiences.

The test has been validated through the years by various means: looking at people’s scores and their likelihood to file a racism-related complaint, cross referencing their perceived experiences with interviews of those who know the respondents well, and looking at other factors tied to perceived racism in past studies, including levels of overall stress and how much a person smokes.

A high score on the Everyday Discrimination Scale turns out to correlate with a wide range of physical ailments. The Jackson Heart Study, a major initiative of the National Institutes of Health in Jackson, Mississippi, has been looking at the reasons behind racial cardiovascular disparities since 1998. Last year, the study published an analysis showing that a higher perceived exposure to discrimination was linked to a 34% higher risk of developing hypertension. The researchers ruled out other causes by controlling data for obesity, exercise, use of substances and measures of socioeconomic status, among other factors.

If the experience of racism causes physical harms, what mechanisms are at work? One arm of research is looking into racism and “weathering,” or accelerated aging. Every person has chronological age—a count of the days they’ve been alive—as well as a biological age, based on physiological biomarkers such as cardiovascular function, metabolism and lung function. A major factor in accelerated aging is chronic stress, which would make it a prime target in studies of how racism affects the body, says Sarah Forrester, assistant professor in the Department of Population and Quantitative Health Sciences at University of Massachusetts Medical School.

Forrester’s group published a study in January showing how accelerated aging might connect racism and poor health outcomes. They followed 3,000 people with a mean age of 45 for 10 years and measured reported levels of discrimination throughout the study. Those who felt more discrimination at the start of the study were more likely to have had a stroke in the follow-up period.

While not everyone who felt racist treatment had markers of accelerated aging, the numbers were telling among those who had had a stroke. Stroke patients who had also reported feeling discriminated against were much more likely to have a higher biological age, a relationship that explained 40% of the variation between all stroke victims in the study, Forrester says.

One biological mechanism underlying the link between racism and health may be eroded telomeres—caps of DNA at the end of chromosomes that protect it so cells can continue dividing in a healthy way. Telomere length shortens as part of the natural aging process, and a pivotal 2014 study showed that experiencing racial discrimination was associated with shorter telomeres.

“It’s an example of the more subtle way racism affects our health, through this gradual process of wear and tear on the body,” says David Chae, associate dean for research at the School of Public Health and Tropical Medicine at Tulane University and the lead author of the 2014 study. “It’s the everyday experiences of being treated with less courtesy, getting worse service or being disrespected,” Chae says. “These types of experiences accumulate and gradually wear down our biology.”

Black Americans who try to overcome the racism in their educational and professional lives may be most affected by these processes. “John Henryism” is a concept coined in the early 1980s by Sherman James, professor emeritus of public policy at Duke University’s Sanford School of Public Policy. In American folklore, John Henry worked on a tunneling crew and went head-to-head with a steam-powered machine, only to die after he had won the contest—a metaphor for Black people who drive themselves to exhaustion in their daily efforts to push themselves past the restrictions placed on them.

A number of studies have found support for the idea that John Henryism wears down the body. Research from the Harvard School of Public Health tracked sleep patterns in workers from different industries and found that Black workers overall were 9% more likely than white workers to get “short sleep”—fewer hours than average. But as white people moved up the professional ladder, they generally got more sleep. Black people did the opposite, sleeping less as they were promoted.

“It very well may be that the Black people had to work much harder to attain and maintain those higher professional roles—and their sleep suffered as a consequence of those complex influences of historical and contemporary racism,” says study coauthor Susan Redline, senior physician of sleep medicine at Brigham and Women’s Hospital in Boston. That lack of sleep may add yet another layer to the wear of perceived racism on the body.

Other research seems to bolster this idea. A 2013 study from the University of Georgia followed Black youth from adolescence to age 19. Students whose teachers rated them as having higher levels of self-control and competence also had higher levels of allostatic load—a measure of physiological wear and tear on the body. Another study, in 2016, found that African Americans who reported higher levels of education also reported higher levels of racial discrimination, a data point that was also linked to a higher incidence of depression.

Researchers are looking at other ways to investigate this problem, and both Forrester’s team and the Jackson Heart Study are incorporating John Henryism measures in new research, including a planned study that will analyze the connection between perceived exposure to discrimination and John Henryism on hypertension.

As such research continues, researchers are beginning to consider how to protect minority communities from the negative impact of racism on health. Interventions to counter health care disparities—improving health screenings and access to care or working to remove the income gap between white and Black workers—may help remedy disparities in a piecemeal way, Forrester says, but they’re not going to solve the problem entirely. “The ultimate solution would be to address underlying racism itself,” she says, “and to have society in general see that all races are as important as white people. Until that happens, we’re going to continue to see what we have now—social inequalities turning into health inequalities.”