The Southeastern United States has a rate of stroke higher than that of the rest of the country, but it is only partly explained by known risk factors like high blood pressure and obesity. Now researchers have found a risk factor independent of the others: living in the Southeast as a teenager.

Writing online in in Neurology, investigators studied a group of 24,545 people, average age 65, for almost six years, recording their medical histories and time spent living in the Southeast, sometimes called the Stroke Belt.

As they controlled for more and more known risk factors, including age, sex, race, blood pressure, socioeconomic level, smoking and diabetes, the association of stroke with living in the Southeast began to weaken. But one association remained statistically significant: Living in the region between ages 13 and 18 increased the risk for stroke by 17 percent.

“We don’t know what it is about the Stroke Belt,” said the lead author, Virginia J. Howard, a professor of epidemiology at the University of Alabama at Birmingham. “It could be factors we haven’t been able to examine sufficiently. We need to do more research.”

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By NICHOLAS BAKALAR