Life expectancies across the globe are projected to rise by an average of 4.4 years over the next two decades, but a study recently published in The Lancet predicts the United States will linger far behind other high-income nations, reaching an average lifespan of just 79.8 years by 2040. Comparatively, frontrunner Spain is forecast to boast an average lifespan of 85.8 years, while Japan sits at a close second with an expected lifespan of 85.7 years.

Newsweek’s Daniel Moritz-Rabson reports that the new rankings find the U.S. dropping from 43rd to 64th place. This staggering 21-spot plunge represents the largest decrease for a high-income nation and suggests that Americans born in 2040 won’t live much longer than those born in 2016. As Ed Cara notes for Gizmodo, average life expectancy in 2016 was 78.7, just 1.1 fewer years than the 2040 projection.

The study, which was led by researchers at the University of Washington’s Institute for Health Metrics and Evaluation (IHME), drew on data from the 2016 Global Burden of Disease study to predict life expectancy in 195 countries and territories. Spain, formerly in fourth place, edged out Japan to nab first, while Singapore (85.4), Switzerland (85.2) and Portugal (84.5) rounded out the remaining spots in the top five.

According to Agence France Presse, the United States’ decline sees it effectively switch places with China. Now in 39th place thanks to an average lifespan of 81.9 years, the Asian powerhouse formerly stood at a lowly 68th.

Other nations projected to enjoy rising life expectancies include Portugal, which jumped from 23rd to fifth after adding 3.6 years to its average lifespan, and Syria, which will move from 137th to 80th by extending its average lifespan from 68.2 years to 78.6 years—assuming, of course, that the country’s devastating civil war soon draws to a close.

At the opposite end of the spectrum, CNN’s Rob Picheta writes that the landlocked African country of Lesotho stands in last place with a predicted life expectancy of 57.3 years. In total, 18 African nations sit at the bottom of the rankings despite seeing lifespan rises between 6.4 and 9.5 years.

"Inequalities will continue to be large," IHME Director Christopher Murray said in a statement. "In a substantial number of countries, too many people will continue earning relatively low incomes, remain poorly educated, and die prematurely. But nations could make faster progress by helping people tackle the major risks, especially smoking and poor diet."

The top determinants of average lifespan are so-called “lifestyle” diseases, according to AFP. These include high blood pressure, obesity, high blood sugar and alcohol and tobacco use. Air pollution, which the team estimates is responsible for taking a million lives in China every year, is another key influence.

In general, scientists expect mortality drivers to shift from infectious diseases like malaria to chronic and non-communicable disorders such as diabetes, lung cancer and kidney disease.

CNN’s Picheta points out that U.S. life expectancy has actually declined over the past two years, in part because of the country’s ongoing opioid crisis, which claimed 63,600 lives in 2016. Obesity also poses a threat to residents, affecting four in every 10 adults and 18.5 percent of children.

Lifestyle changes could help offset these issues, Brett Molina writes for USA Today. A June report by the Centers for Disease Control and Prevention found that only 23 percent of U.S. adults get enough exercise, while a 2017 study reported just one in 10 Americans eats a sufficient amount of fruits and vegetables.

The team’s findings aren’t set in stone. In fact, the researchers mapped both best- and worst-case scenarios. In the former, 158 countries experienced life expectancy gains of at least five years, while 46 saw gains of 10 years or more. In the latter, nearly half of all countries saw a decrease in life expectancy, with lowest-ranked Lesotho standing at just 45.3 years.

“The future of the world’s health is not pre-ordained, and there is a wide range of plausible trajectories,” lead author Kyle Foreman, director of data science at IHME, said in a statement. “But whether we see significant progress or stagnation depends on how well or poorly health systems address key health drivers.”