POPULATION forecasting is not simple. Demographers use mortality data—information about when people die and why—to estimate the likely life expectancy of people still alive. The UN, the World Health Organisation (WHO) and others produce periodic forecasts that are fairly similar. Wealthy countries such as Japan, Switzerland and Australia have the highest life expectancies, though the estimates vary slightly depending on the methodology used. But a new study of 35 rich countries by researchers at Imperial College London and the WHO, and published in the Lancet, a medical journal, uses a combination of 21 statistical models, instead of just one. The results, say the authors, are more reliable. They are also surprising.

By 2030, they think, South Korea will have seen the biggest gains in life expectancy for both men and women. A girl born there in 2030 is likely to live past her 90th birthday, seven years longer than one born in 2010. South Korean men are expected to live to just over 84, leapfrogging 18 other countries to the top of the ranking.

South Korea’s trajectory has been remarkable. Its annual GDP per person is more than 20 times the level in 1960. America and Britain saw only a trebling over the same period. These economic gains have improved the nation’s health. In 1985 life expectancy in South Korea was more than four years lower than in America for both sexes; by 2010 it had caught up for men and risen to three years higher for women. The main reasons were big reductions in child mortality, deaths from infections and deaths from diseases related to high blood pressure, which include cardiovascular disease and stroke. The share of South Koreans who have raised blood pressure is lower than in any other country. Few South Koreans suffer from obesity, which increases the chance of several chronic illnesses, including diabetes, heart problems and cancer. Smoking rates among women remain low.

Other countries expected to do well are improving for similar reasons. Few women in France and Switzerland are obese. By 2030 Hungarian men are expected to live 7.5 years longer on average than they did in 2010, partly because fewer drink too much or smoke. New Zealand, Australia and Canada have low rates of infant mortality and death from traffic accidents, and good records in preventing and treating heart disease and cancer.

But the authors predict far less progress elsewhere. In some less wealthy countries, such as Macedonia and Serbia, average life expectancies are low and predicted to improve only modestly. Sweden and Japan can also expect relatively small gains, though that may be unavoidable since they already sit near the top of the table.

The most striking outlier is America. The world′s biggest economy is among the bottom five countries in expected gains for both men and women. At 77 years for men and 81 years for women in 2010, American life expectancy is already among the lowest in the rich world. That is partly because America is the only country in the OECD that does not have universal health care, meaning poorer health for poorer people. It also has the highest maternal and child mortality rates, so fewer people reach old age. High obesity and homicide rates shorten lifespans as well.

As a result, the study predicts that life expectancies in far poorer countries will eventually overtake America′s. It finds that men born in the Czech Republic will live as long as those born in America by 2030, as will women in Croatia and Mexico. Moreover, projections of life expectancy in America may need to be revised downwards. The researchers use actual American mortality data to 2013, with forecasts beyond that point. In fact, American life expectancy fell in 2015. If Republican plans to repeal Barack Obama′s health law end up reducing access to care, this trend might accelerate.

As recently as 2000 it was thought impossible that a country’s average life expectancy would ever exceed 90. That so many developed countries can expect people to live beyond 80 is testament to health-care successes. The next challenge for governments will be to ensure there are sufficient health and social care policies in place to support fast-swelling older populations.