The World Values Survey, the work of a global network of social scientists, says happiness increased from 1981 to 2007 in 45 of 52 countries.

"Rich is better than poor, but more importantly being free -- being in a democracy -- as well as gender equality and tolerance of minorities were the key factors to happiness," study director Ronald Inglehart of the University of Michigan's Institute for Social Research, told United Press International. "The most important determinant of happiness is the extent to which people have free choice in how to live their lives."

Denmark tops the list of surveyed nations, along with Puerto Rico and Colombia. A dozen other countries, including Ireland, Switzerland, the Netherlands, Canada and Sweden also rank above the United States. Three of the world's poorer countries with long histories of repressive government -- Moldova, Armenia and Zimbabwe -- are at the bottom of the list.

The survey, published in the journal Perspectives on Psychological Science, had a representative sample of about 1,400 in each country and the margin of error of 3 percentage points to 5 percentage points.