As the gap between rich and poor grows in the United States, so, too, does the gap in life expectancy, in most parts of the country.  

Generally, people with high incomes live longer than those with low incomes. That gap is wider in some places and narrower in others, such as New York where the life expectancy gap is decreasing rather than increasing, according to a new report in the Journal of the American Medical Association released on Monday.

The report authors compared income levels and death certificates nationwide between the years 2001 and 2014.

In metropolitan area counties, 40-year-olds with income below $28,000 now have a life expectancy between 80 and 83 years, between two and six years shorter than that of the top 1 percent. In some counties in other regions, the wealthiest are likely to live a decade longer than the poorest.

Dana March, an epidemiologist at the Mailman School of Public Health, said in New York City at least some of the credit should go to the activist health policies instituted by former Mayor Michael Bloomberg and continued by the current mayor. 

"We have a ban on smoking [in public places], some of the highest tobacco taxes in the country, a ban on transfats," said March, who was not involved in the multi-author study. "And one of the especially interesting things that is that you wouldn't expect much of the improvement [from lower rates of cancer and heart disease] to show up statistically for many years."

As a result, March said, there could be further gains in life expectancy among the poor in this region in future years.

She also cautioned that the study had limits. She said that because it only compared counties across the country (including the city's five boroughs), the relatively large geographical areas could mask deep and persistent disparities.

"If we looked at the 'bottom of the bottom' in New York City, I don't know that we'd see those radical gains," March said. "To my mind, there's no possible way that the bottom 1 percent of New Yorkers have made the kinds of gains that the rest of this [broader] swathe of the impoverished have."