FINANCE   AND   DEVELOPMENT      People around the world share their thoughts on what sparks this elusive feeling                 Economists generally measure a country’s success in terms of its GDP. But GDP doesn’t always accurately measure well-being. And even high per capita income doesn’t necessarily guarantee a happy population, economist Richard Easterlin discovered in the 1970s. While rich people are generally happier than poor people in a given country, Easterlin found, richer countries aren’t always happier than poorer ones. And for individuals, higher income fails to increase happiness beyond a certain level, some believe. According to Nobel laureates Angus Deaton and Daniel Kahneman, that threshold for the United States is $75,000—though in s...