For poor Americans, the place they call home can be a matter of life or death.

The poor in some cities — big ones like New York and Los Angeles, and also quite a few smaller ones like Birmingham, Ala. — live nearly as long as their middle-class neighbors or have seen rising life expectancy in the 21st century. But in some other parts of the country, adults with the lowest incomes die on average as young as people in much poorer nations like Rwanda, and their life spans are getting shorter.

In those differences, documented in sweeping new research, lies an optimistic message: The right mix of steps to improve habits and public health could help people live longer, regardless of how much money they make.

One conclusion from this work, published on Monday in The Journal of the American Medical Association, is that the gap in life spans between rich and poor widened from 2001 to 2014. The top 1 percent in income among American men live 15 years longer than the poorest 1 percent; for women, the gap is 10 years. These rich Americans have gained three years of longevity just in this century. They live longer almost without regard to where they live. Poor Americans had very little gain as a whole, with big differences among different places.

But the fact that some places have increased the life span of their poorest residents suggests that improving public health doesn’t require first fixing the broader, multidecade problem of income inequality. Small-scale, local policies to help the poor adopt and maintain healthier habits may succeed in extending their lives, regardless of what happens with trends in income inequality.

“You want to think about this problem at a more local level than you might have before,” said Raj Chetty, a Stanford economist who is the study’s lead author.

“You don’t want to just think about why things are going badly for the poor in America. You want to think specifically about why they’re going poorly in Tulsa and Detroit,” he said, naming two cities with the lowest levels of life expectancy among low-income residents.

The research, in the works for nearly three years and based on a vast trove of records on earnings and deaths, is the most detailed analysis to date of a pattern first identified at least a couple of centuries ago, that more money translates into a longer life.

It could be as simple as this: Wealth buys higher-quality medical care, which allows people to live into old age. But a long line of evidence, including the new work, suggests it’s less obvious than it might seem. The affluent seem to live in healthier ways. They exercise more, smoke less, feel less stress and are less likely to be obese.

It’s not even certain that the cause and effect flows from higher income to greater health; to some degree, it may go the other direction as well, because people who are healthy are better able to hold down a demanding job, and so have higher incomes.

The new paper, in fact, finds little correlation between a region’s Medicare spending rate or the proportion of the population with health insurance and how long its poor citizens live.

Public health experts who examined the results said the weak relationship did not mean that health insurance had no value. Research has long established that health care interventions have a much smaller effect on life span than behavioral factors like smoking and exercise. But health care does help people who are already sick lead healthier lives. And it can provide economic security and peace of mind that improve the lives of the poor in other ways.

Economic measures like the unemployment rate and income inequality also showed little relationship to low-income people’s life spans. There was a much stronger relationship between longevity and obesity and smoking rates, which is unsurprising. Places where poor citizens had long life spans also tended to have a high concentration of college graduates and high local government spending.

Life expectancy for the poor is lowest in a large swath that cuts through the middle of the country, and it appears in pockets in the rest of the country, in places like Nevada. David M. Cutler, a Harvard economist and an author of the paper, calls it the “drug overdose belt,” because the area matches in part a map of where the nation’s opioid epidemic is concentrated.

The new findings dovetail with a much-discussed paper by Anne Case and Angus Deaton published last year. That research showed rising death rates among middle-age white Americans, especially those with low education. It also showed a sharp increase in drug and alcohol poisonings, suicides and accidents in the first years of this century. Research from the Brookings Institution published in February also found a growing gap in life span between the rich and the poor.

“There is some deeper distress going on among white middle-aged Americans that may continue to propel these mortality rates higher,” Mr. Deaton, a Princeton economist who wrote an editorial critiquing the new paper by Mr. Chetty and his colleagues, said in an interview. “If so, these people at the bottom will live even less long than they’re calculating.”

The great question for public health officials is what strategies might help low-income people live as long as their richer neighbors.

“There is a very strong correlation between income and life span,” Dr. Thomas R. Frieden, director of the Centers for Disease Control and Prevention, said in an interview. “But it is not inevitable. There are things we can do to change the life trajectory of people. What improves health in a community? It includes wide access to social, educational and economic opportunity.”

A common thread among many of the places with a smaller longevity gap was population density, with wealthy cities leading the way. New York has a high rate of social spending for low-income residents andhas been aggressive in regulating trans fats and smoking.

In the area in and around Birmingham, Ala., the life span for adults in the bottom quarter of income rose 3.8 years for men and 2.2 years for women from 2001 to 2014. (Because people of different races have different life expectancies regardless of income, the researchers statistically adjusted these local numbers to simulate a world in which all places matched the racial composition of the country as a whole. These numbers are after these race adjustments.)

Dr. Mark E. Wilson, chief executive of the health department in Jefferson County, Ala., which includes Birmingham, ticked off a number of things that might have helped.

The county expanded availability of preventive health care like vaccinations and mammograms by opening clinics in poorer neighborhoods in the 1990s and early 2000s (though recently it has closed some of the clinics). Although a relatively high percentage of the population lacks health insurance, a portion of local taxes goes to hospital care for those who cannot pay. The county has been ahead of the rest of Alabama in banning smoking in restaurants and workplaces, with a law enacted in 2012. And philanthropic foundations backed by old industrial money have funded campaigns to make people healthier in the Birmingham area.

“These aren’t all huge-scale projects, but there is still an alignment of getting resources moving in the same direction around health,” Dr. Wilson said. “We’re trying to establish a culture of health and get it more and more on the radar screen of our community.”

Mr. Cutler, the Harvard economist, argues that the new research should serve as a jumping-off point.

“Why is it that Birmingham has done well but Tulsa has done poorly?” he said.

It may be good to know that poor Americans are living a lot longer in some places than in others, but it would be better to know — in terms of specific policy prescriptions — how the places with better results are doing it.