The gap in life expectancy between rich and poor Americans has grown wider since the start of the century, with the wealthiest men outliving the most destitute men by nearly 15 years, according to a study published on Monday.

The average lifespans of U.S. men and women in the top 5 percent income bracket rose by more than two years between 2001 and 2014, while at the bottom 5 percent, life expectancy nearly stagnated, researchers found.

Differences in lifespan were most pronounced in comparisons of the extremely wealthy and extremely poor, according to the study analyzing tax and death records. It was published in the Journal of the American Association Medical Association.

In the top 1 percent, men and women outlived the bottom 1 percent by 14.6 and 10.1 years respectively, it said.

U.S. men in the bottom 1 percent have life expectancies similar to men in developing countries such as Sudan and Pakistan, it said.

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The researchers said they hoped their findings would help shape public health policy and narrow the life expectancy gap.

"The magnitude of socioeconomic gaps in life expectancy, how these gaps are changing over time, and their determinants remain debated," they wrote.

Where poorer people lived within the United States was a factor as well in how long they lived, the research found.

The study identified ten states where low-income residents have the shortest live expectancies, including an eight-state "geographic belt from Michigan to Kansas" in the middle of the country.

The states of Nevada, Indiana and Oklahoma trailed the country, with a 77.9-year life expectancy average.

California, New York and Vermont offered the longest low-income life expectancy at 80.6 years.

Other factors seen to account for how long poorer Americans lived included smoking and obesity, it said.

The study found that neither medical care nor the ratio of those insured affected life expectancy for poorer individuals.