Life expectancy is a commonly used measure to describe the health of a population.

A recent report from the U.S. Centers for Disease Control and Prevention (CDC) shows life expectancy has dropped in the United States for the first time in decades.

The average life expectancy for someone born in the United States in 2015 fell to 78.8 years.

The last time it dropped was during the AIDS epidemic in the early 1990s, and researchers are at a loss to pinpoint the exact cause of the decline this time around.

But what influences life expectancy in the first place?

Researchers say it’s not just family history that can determine how long you’ll live.

It’s also your ethnicity, socioeconomic status, and even what part of the country you live in.

Michael Stepner from the Massachusetts Institute of Technology is the senior researcher for a study released earlier this year that used data from 1.4 billion anonymous earnings and mortality records to measure differences in life expectancy by income areas.

“Low-income Americans are living much shorter lives than their richer counterparts. The richest one percent of Americans lives 10 to 15 years longer on average than the poorest one percent,” Stepner told Healthline. “The gap between the rich and the poor has been growing rapidly. The richest Americans gained three years of life expectancy between 2001 and 2014, while the poorest Americans experienced no gains.”

To put that last figure in perspective, Stepner says, the CDC estimates that curing cancer would raise life expectancy in America by three years. Therefore, the change seen in the past 15 years is equivalent to the richest Americans winning the war on cancer.

Stepner and his colleagues found there was significant variation in life expectancy for low-income earners across the country.

“We found that low-income Americans live longer in affluent cities — places with dense populations, high average home values, and high government expenditures,” he said.

What researchers are yet to establish is why people live longer in those places.

“It could be that affluent cities have more effective public health policies, like bans on indoor smoking, or greater funding for public services that improve local health. It could be a social dynamic, where low-income people living in affluent areas are adopting healthy behaviors,” Stepner said.

Dr. Seth Berkowitz from Massachusetts General Hospital says there is considerable evidence to suggest a “social safety net” is effective in combatting the adverse effects of poverty.

“For a number of reasons, this safety net is likely to be easier to maintain in more affluent and densely populated areas,” he told Healthline. “However, that doesn’t mean we don’t need to work hard to help protect those who aren’t living in such areas from the adverse effects of poverty … I think we need to work to maintain the social safety net we have, and improve it where needed.”

However, the safety net isn’t effective everywhere, and unmet socioeconomic needs continue to negatively influence health outcomes and life expectancy.

“One thing that people often take for granted is the interrelatedness of many needs. If someone does not have enough money for food, that can cause many trade-offs as people seek to stretch their resources. For example, someone may take less of their prescribed medications, something we call the ‘treat or eat’ phenomenon,” he said.

Berkowitz says those from racial or ethnic minorities, particular those who identify as non-Hispanic African-Americans or Hispanic have greater unmet needs for basic resources like nutritious food.

“Why this happens is not completely clear, but likely has to do both with discrimination and structural socioeconomic factors,” he said.

From 2004-2014 life expectancy in the United States increased more for African-Americans than the white population, narrowing the difference to 3.4 years in 2014.

Despite this, disparities in health and life expectancy still exist.

A 2016 report on the state of obesity in the United States noted neighborhoods with higher rates of ethnic and racial minorities had less access to healthy, affordable food options, as well as limited access to safe places to be physically active.

According to the CDC more than one third of U.S. adults are obese. Non-Hispanic African-Americans have the highest rates of obesity, at a rate of 48 percent, followed by Hispanics at 42 percent.

More than 29 million Americans live in “food deserts,” where they don’t have access to supermarkets within a mile of their home in urban areas or within 10 miles in rural areas. This makes it challenging to access affordable food that is healthy.

It is a problem both Stepner and Berkowitz argue is significantly contributing to poorer health outcomes and shorter life expectancies.

“The fact that nutritious food is often more expensive and less available than less nutritious food is a major barrier to maintaining good health for many people,” Berkowitz said.

“One pattern that comes through loud and clear,” added Stepner, “is that places with high smoking rates, high obesity rates, and lower rates of exercise have shorter life expectancies. This pattern suggests that differences in health behaviors are an important component of why Americans with low incomes live longer in some places, and shorter in other places.”

Berkowitz suggests a multifaceted approach is the only way to overcome such health inequalities.

“Ensuring that everyone can access nutritious foods and has appropriate health education is a start, but addressing structural factors such as neighborhood safety and walkability is also important, as is understanding what our food system incentivizes,” he said.

Stepner acknowledges there are many cities in America where the gaps in life expectancy between the rich and poor are small, or shrinking. But he says the degree of inequality across the United States is striking.

His study noted the poorest men in America have a life expectancy comparable to men in Sudan or Pakistan.

“It’s yet another sign of the extent of inequality in America,” Stepner said. “The average American man is richer, and lives much longer, than an average man in Sudan or Pakistan. But there are also Americans who are struggling.”

“Despite living in a rich country,” he added, “there are lots of Americans struggling to live full lives. Shorter life expectancies are a particularly heartbreaking aspect of that inequality.”

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