We all live longer when times are good, right? Not so, according to a new study which says that in developed countries, the elderly have a higher mortality rate when the economy goes into higher gear.

Even its authors are baffled by the outcome.

The finding was "highly unexpected", Mr Herbert Rolden from the Leyden Academy on Vitality and Ageing in the Netherlands, told AFP.

In the long term, economic prosperity is credited with lower mortality rates across all age groups - largely due to a drop in old-age mortality.

But the picture changes when you look at short-term economic fluctuations, according to the study which appears in the Journal of Epidemiology and Community Health.

For every rise of one percentage point in a country's gross domestic product, mortality among 70-74-year-old men rose by 0.36 per cent and for women of the same age by 0.18 per cent, it found.

Among 40-45-year-olds, the corresponding rise was 0.38 per cent for men and 0.16 for women.

The study analysed mortality and economic growth figures from 1950 to 2008 in 19 developed countries - Australia, Japan, New Zealand, the United States and several in Europe.

"Since many developed countries are currently in a recession, one could expect that this has a dampening effect on old age survival," says the study.

"However, it has been found that annual increases in unemployment, or decreases in gross domestic product (GDP) are associated with LOWER mortality rates." A similar, seemingly counterintuitive trend had already been found in younger people.

That had been ascribed to more work stress and traffic accidents due to higher employment in economic boom times.

But such factors are unlikely to hold true for older, retired people, said Mr Rolden.

"We are still in the dark on what really explains the association," he admitted.

The cause may lie in a change in social structure, with younger relatives and friends working longer and having less time to care for the elderly, according to one, untested, theory.

Another idea pins the blame on air pollution, which increases during economic expansion and is likelier to have more of an effect on frail people.

The team call for more research. Unravelling the mystery could have many benefits, they say.