The world is becoming a happier place according to a study published in this month's Perspectives of Psychological Science.
Data from national surveys conducted between 1981 and 2006, which were collated by researchers at the University of Michigan Institute for Social Research, showed that happiness was on the rise in 40 out of 52 countries.
And in a separate happiness ranking, which looked at 97 countries representing 90 percent of the world's population, only 20 countries were listed as unhappy.
University of Michigan political scientist Ronald Inglehart, the lead author of the study, said the upswell in happiness came as a surprise to researchers, who have long felt it was "almost impossible to raise an entire country's happiness level."
"There has been a lot of research over the last 25 years indicating that happiness is very stable," Inglehart told AFP.
"There may be short-term changes but it returns to a set point," he said.
But the study, which was part of the ongoing World Values Surveys, appeared to disprove that theory.
For the past 26 years, World Values Surveys have asked more than 350,000 people how happy they are.
Among the 52 countries and territories for which long-term comparative data were available, India, Ireland, Mexico, Puerto Rico and South Korea showed steep upticks in happiness last year, while the happiness quotient in 14 other countries, including nine in Europe, also rose, but less sharply.
Those 14 countries are Argentina, Canada, China, Denmark, Finland, France, Italy, Japan, Luxembourg, the Netherlands, Poland, South Africa, Spain and Sweden.
"Economic growth, democratization and tolerance are strongly linked with happiness," said Inglehart.
"We have had an unusual set of circumstances in the last 25 years where all of these things that are quite important and have strong linkages to happiness have been going in a favorable direction. So most countries have rising levels of happiness," he told AFP.
"Democracies are significantly happier than non-democratic countries; prosperous countries tend to be happier than poor countries; and tolerant people -- even intolerant people living in a tolerant society -- tend to be happier."
In the United States, Switzerland and Norway, happiness was stagnant, but all three countries were still in the top 20 of the 97 nations that were ranked in order of happiness levels.
Denmark, where 52 percent of the population said they were very happy, was at the top of that list and Zimbabwe at the bottom, with only around four percent of Zimbabweans saying they were happy.
"Zimbabwe has everything going wrong. It's desperately poor, AIDS is high, people are being killed, the political system is repressive. It's not a great place to live these days and it's deeply unhappy," said Inglehart.
"The results clearly show that the happiest societies are those that allow people the freedom to choose how to live their lives," Inglehart said, citing the tolerant societies and democratic political systems in Denmark, Iceland, Switzerland, the Netherlands and Canada -- all of which rank among the 10 happiest countries in the world.