After analyzing records from every U.S. county between 1980 and 2014, Christopher Murray, head of the Institute for Health Metrics and Evaluation at the University of Washington, and his team found that life expectancy can vary by more than 20 years from county to county. "In counties with the longest lifespans, people tended to live about 87 years, while people in places with the shortest lifespans typically made it only about 67," reports NPR. From the report: The discrepancy is equivalent to the difference between the low-income parts of the developing world and countries with high incomes, Murray notes. For example, it's about the same gap as the difference between people living in Japan, which is among countries with the longest lifespans, and India, which has one of the shortest, Murray says. The U.S. counties with the longest life expectancy are places like Marin County, Calif., and Summit County, Colo. -- communities that are well-off and more highly educated. Counties with the shortest life expectancy tend to have communities that are poorer and less educated. The lowest is in Oglala Lakota County, S.D., which includes the Pine Ridge Native American reservation. Many of the other counties with the lowest life expectancy are clustered along the lower Mississippi River Valley as well as parts of West Virginia and Kentucky, according to the analysis. There's no sign of the gap closing. In fact, it's appears to be widening. Between 1980 and 2014, the gap between the highest and lowest lifespans increased by about two years. The reasons for the gap are complicated. But it looks like the counties with the lowest lifespans haven't made much progress fighting significant health problems such as smoking and obesity. The study has been published in the journal JAMA Internal Medicine.