A new statistical model predicts that by 2100 the number of people older than 85 worldwide will increase more than previously estimated, and there will be fewer working-age adults to support them than previously expected.
The findings, reported by researchers at the University of Washington and the United Nations, suggest an even greater decrease in the coming decades in support for social security programs for elderly adults.
Lead author Adrian Raftery, UW professor of statistics and sociology, was surprised by how dramatically the proportion of the world's "oldest old" will increase by the end of the century.
"This has been studied a lot in developed countries, but what we see with this model is that the increase in people over 85 will be a worldwide phenomenon," he said.
The study will be published Aug. 20 in the online early edition of the Proceedings of the National Academy of Sciences.
In China, the world's most-populous country, the number of working-age adults for each person 65 or older will shrink from 7.9 in 2010 to 1.6 in 2100. The ratio in India, the world's second-most-populous country, will decrease from 11.1 in 2010 to 2.0 in 2100.
The United States' ratio declines from 4.6 in 2010 to 1.8 by the end of the century. Other developed nations with low fertility rates show somewhat larger declines, including the Netherlands dropping from 4.0 to 1.6 and the United Kingdom dropping from 3.6 to 1.6 by century's end.
"The United States has more favorable numbers than other developed countries now, and will retain a slight advantage over other countries at the end of the century," Raftery said. He attributes the United States' relatively more-promising outlook to the country's higher levels of new births and to immigration.
The researchers did not produce population predictions for the 38 countries with generalized HIV/AIDS epidemics, because those nations require a different statistical method.