South Dakota's Oglala Lakota County and Colorado's Summit County are only separated by about 400 miles, but when it comes to average life expectancy of residents, the two are light years apart.

According to the newly released "Inequalities in Life Expectancy Among US Counties, 1980 to 2014" from the Journal of the American Medical Association, the two are at opposite ends of the spectrum when it comes to how long, on average, its residents will live.

Oglala Lakota County (formerly known as Shannon County) in the Pine Ridge Reservation in southwestern South Dakota is at the bottom of the scale, with an average life expectancy of only 66 years. Summit, in the mountains of Central Colorado, is at the top, with an average life expectancy of close to 87 years.

So why the big disparity? Economics is probably the biggest reason.

According to the latest figures, the per capita income for Oglala Lakota County was $6,286 with about 45% of families and 52% of the total population below the poverty line. In Summit County, the per capita income was $28,676 with about 3% of families and 9% of the population were below the poverty line.

In its' conclusion, the JAMA cites other factors that contribute to the variance in numbers:

Geographic disparities in life expectancy among counties are large and increasing. Much of the variation in life expectancy among counties can be explained by a combination of socioeconomic and race/ethnicity factors, behavioral and metabolic risk factors, and health care factors. Policy action targeting socioeconomic factors and behavioral and metabolic risk factors may help reverse the trend of increasing disparities in life expectancy in the United States.

As a state, South Dakota is in the upper half of life expectancy, ranking 21st overall with an average life expectancy of 79.5 years.

Hawaii is number one (81.3 years). Mississippi is last (75 years).

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