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Happiness nose dives as you hit middle age - but only if you live in the affluent West, according to experts.

Using world survey data, the Lancet study authors found life satisfaction followed a predictable trajectory depending on where people lived.

In countries such as the UK and the US, life satisfaction followed a U-shape, dipping to a low in midlife.

In Africa it was low throughout, and in Eastern Europe, the former Soviet Union and Latin America it declined with age.

The reasons behind the trends were multiple and complex, but there were some plausible explanations as well as important lessons to be learned, said Prof Andrew Steptoe, of University College London, and colleagues.

Their report is part of a series on health and ageing, published in The Lancet, warning of a growing burden of chronic disease and diminished wellbeing affecting the quality of life of older people.

Satisfaction

The researchers used four years of data from the Gallup World Poll in more than 160 countries covering more than 98% of the world's population, to make their evaluations.

As well as physical health and pain, they considered three measures of wellbeing:

evaluative - how satisfied people were with their lives

hedonic - feelings or moods such as happiness, sadness and anger

eudemonic - judgements about the meaning and purpose of life

In Western world nations, life satisfaction bottomed out between the ages of 45 and 54 before rising again.

Co-researcher Angus Deaton, of Princeton University in the US, said economics could explain this.

"This is the period at which wage rates typically peak and is the best time to work and earn the most, even at the expense of present wellbeing, so as to have increased wealth and wellbeing later in life," he said.

It might explain why elderly people were happier despite becoming more frail - although poorer health led to lower ratings of life satisfaction among the elderly, higher life satisfaction seemed to stave off physical health declines.

It would appear that wellbeing goes along with economic prosperity in the world Prof Andrew Steptoe

In transition countries, such as the former Soviet Union, life satisfaction declined steadily with age and was generally lower overall than in the West.

This could again be linked to economics, said the researchers. Older people in these countries had lost a system that, however imperfect, had given meaning to their lives, and, in some cases, their pensions and their health care.

In sub-Saharan Africa, satisfaction was very low throughout life, while stress and worry was high.

"The findings undoubtedly show the recent experiences of the region and the distress that these events have brought to older people," said Prof Deaton.

Prof Steptoe said while money did not equate to happiness, economic progress did promote wellbeing to an extent.

"It would appear that wellbeing goes along with economic prosperity in the world," he said.