Life expectancy has steadily climbed for decades. But in the U.S., these gains are threatened by unequal access to healthcare.

A sizable feather in modern medicine’s cap is the fact that life expectancy has risen consistently in the developed world since before World War II.

But that feather is dangerously close to blowing away in the wind, according to new population research. The trouble isn’t that medicine has fallen behind; it’s that medical advances don’t reach everyone equally.

Life expectancy, which currently stands at about 78 years in the United States, is just an average — there are people who die later and others who die earlier. The gap between those early and late deaths is wider in the United States than in most other developed countries.

That got Shripad Tuljapurkar, a Stanford University biologist, postdoctoral fellow Meredith Trotter, and Duncan Gillespie, a University of Liverpool public health researcher, thinking about what might cause the gap to expand or contract. They published their findings recently in the journal Demography.

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The longevity gap in the U.S. “seemed surprising to me given how important health is supposed to be in the U.S., and all the improvements that we keep making that are supposed to make people live longer and have fewer accidents,” Tuljapurkar told Healthline.

What makes the U.S. so unequal?

The researchers observed, first, that extending the lives of elderly people doesn’t just increase the country’s average life expectancy. It also increases the gap between early and late deaths, unless doctors also find ways to save more young people.

The age at which deaths “tip” from closing the gap down to widening it is 65, the official start of old age, the paper calculates.

It’s a statistical effect, but it points to a real issue in the U.S. healthcare system. We save ever sicker, older people but have trouble improving outcomes for some younger, healthier people.

“The great good of development has been the consistent improvement of medical technology and care for the elderly, reflected in steadily declining old-age mortality,” the researchers wrote. “However, our results suggest that without comparable progress against young adult mortality, through action on the social determinants, mortality change is unlikely to follow the historical pattern of rising life expectancy and falling lifespan inequality.”

These younger people may not have access to healthcare, or may be exposed to violent crime. The strongest forces pulling young deaths away from the average are heart disease and homicides. Heart disease is widespread in the U.S., and homicides generally target people far younger than the average age of death.

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The biggest problem is lack of access to healthcare, the study suggests. Longevity studies done for the World Health Organization have shown that the U.S.’s average life expectancy of 78 years masks huge differences between poorer and richer counties.

In some U.S. counties, life expectancy is lower than it is in Honduras or the Philippines.

“At the lower end of education and income spectrum, people don’t have regular access to healthcare, so they’ll take medications for a while, then they lose coverage and they can’t afford it, so they just quit,” Tuljapurkar said.

The U.S.’s longevity gap started to grow relative to its neighbor Canada’s in the 1980s, according to the researchers. That’s when Canada launched its national healthcare system to give all its citizens access to care.

“That seemed to us as the simplest explanation that we could come up with,” Tuljapurkar said, “but it’s not a proof in the scientific sense.”

The hypothesis will be tested soon enough, as the Affordable Care Act expands access to health insurance in America. If the law survives legal challenges, Tuljapurkar predicted that by 2020, the number of deaths among younger adults in the U.S. will decline.

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