Money doesn't buy happiness over the long term, a new study has found. The results apply to developed and developing countries worldwide, said study researcher Richard Easterlin, a professor of economics at the University of Southern California. "Happiness doesn't increase with the rate of economic growth even in less-developed countries or transitional countries," Easterlin told LiveScience. "We already know that to be true of developed countries, but now it's been extended to countries of lower levels of income." Easterlin and his colleagues reported the results this week (Dec. 13) in the journal Proceedings of the National Academy of Sciences.