In 1974, economist Richard Easterlin pointed out that beyond a certain point - presumably when people's basic needs for food, shelter, public order and work are met - greater wealth does not generate more national happiness.

The America of 2007 is far richer than the America of 1977. Life expectancy is 78 years, up from 74 years. Our homes are bigger and crammed with more paraphernalia (microwave ovens, personal computers, flat-panel TVs). But happiness is stuck.

In 1977, 35.7 percent of Americans rated themselves "very happy", 53.2 percent "pretty happy" and 11 percent "not too happy", reports the National Opinion Research Center at the University of Chicago. Last year, the figures were similar: 32.4 percent "very happy", 55.9 percent "pretty happy" and 11.7 percent "not too happy". Likewise, in most advanced countries, self-reported happiness has been flat for decades.