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A new group of studies into racial disparities among people with Alzheimer’s disease suggests that social conditions, including the stress of poverty and racism, substantially raise the risks of dementia for black Americans.

In four separate studies, researchers found that conditions that affect blacks disproportionately compared with other groups — such as poor living conditions and stressful events like the loss of a sibling, parents’ divorce or chronic unemployment — have severe consequences for brain health later on.

One study by University of Wisconsin researchers found that stress literally takes years off a person’s life in terms of brain function — an average of four years for blacks compared with 1 ½ years for whites.

Another Wisconsin study showed that living in a disadvantaged neighborhood is associated with later decline in cognitive function and even the biomarkers linked to Alzheimer’s disease, which is the most common form of dementia.

In the other two studies, researchers with Kaiser Permanente and the University of California, San Francisco found a higher degree of dementia risk for people born in states with high rates of infant mortality. Researchers at Kaiser and the University of California, Irvine, found that racial disparities in the incidence of dementia that were previously found among people who are 65 years and older also appear in the very oldest demographic, people who are 90 or more.

These lifelong effects of stress and disadvantage could be direct, perhaps in line with previous research showing that sustained stress can physically alter the brain. Or the impact could be the result of cascading effects, such as when a powerfully disruptive event affects a person’s early schooling and limits achievement later on.

“No one’s looking at the same kind of things, but the research all dovetails really well,” said Megan Zuelsdorff, an epidemiologist with the University of Wisconsin School of Medicine and Public Health. “It is the social environment that’s contributing to disparities.”

She and other researchers said the overall thrust of the studies’ findings suggest the need for more urgent interventions directed at those communities.

“Not one of these things is good news — except that they are modifiable,” Zuelsdorff said.

Over the years, researchers have theorized that blacks are more susceptible to Alzheimer’s owing to genetics and higher rates of obesity, diabetes, hypertension and cardiovascular disease.

But researchers in recent years have also been focused on social factors that might raise the risk. It’s long been known that stress is associated with social disadvantage, and in the United States and other countries, members of minority groups often suffer disproportionally from those disadvantages.

Amy J. Kind, a physician and researcher at the University of Wisconsin, looked to see whether there is a relationship between disadvantaged neighborhoods and the prevalence of dementia. The researchers found that people in the most disadvantaged neighborhoods performed significantly worse in every aspect of cognitive function that was tested; they also had disproportionately higher levels of an Alzheimer’s biomarker.

“This linkage between neighborhood disadvantage and Alzheimer’s has never been explored until our work,” Kind said.

Zuelsdorff’s study found that blacks reported more than 60 percent more stressful events — severe problems in school, family bankruptcy, alcohol-related problems at home — in their lifetimes, and that those experiences were associated with poorer cognitive function.