Key groups of people in the United States have grown happier over the past few decades, while other have become less so. The result: Happiness inequality has decreased since the 1970s, a new study finds.

Previous research has found that happiness is partly inherited, and that it can be highly contagious. So what's the state of glee in these United States? Depends on who and how you ask. One recent study found, for example, that Baby Boomers are not as happy today as other generations in other eras.

In the new study, University of Pennsylvania economists Betsey Stevenson and Justin Wolfers looked at different demographic groups and found that the American population as a whole is no happier than it was three decades ago. But the gap between the happy and the not-so-happy has narrowed significantly.

"Americans are becoming more similar to each other in terms of reported happiness," says Stevenson. "It's an interesting finding because other research shows increasing gaps in income, consumption and leisure time."

Their study is detailed this month in the Journal of Legal Studies.

Who's happy?

Globally, happiness is on the rise, according to a study last year. Denmark is the happiest nation and Zimbabwe is the most glum. The United States ranked 16th.

The U.S. happiness gap between whites and non-whites has narrowed by two-thirds, the new study found. Non-whites report being significantly happier than they were in the early 1970s, while whites are slightly less happy. The happiness gap between men and women closed as well. Women have become less happy, while men are a little more cheerful.

Stevenson and Wolfers used data collected from 1972 to 2006 through the University of Chicago's General Social Survey. Each year, participants were asked, "Taken all together, how would you say things are these days — would you say that you are very happy, pretty happy, or not too happy?"

The proportion of people choosing "pretty happy" has increased from 49 percent in 1972 to 56 percent in 2006. Responses of "very happy" and "not too happy" decreased in relatively equal amounts. This convergence toward the middle response closed happiness gaps in nearly all the demographic groups examined.

"The U.S. population as a whole is not getting happier," Stevenson said. "For every unhappy person who became happier, there's someone on the other side coming down."

What's going on?

The authors say that it's hard to pin down what exactly is causing the narrowing happiness gap. But they suggest that money probably is not the answer. In fact one recent study found that giving money away generates happiness.

"That these trends differ from trends in both income growth and income inequality suggests that a useful explanation may lie in the nonpecuniary domain," they write.

One demographic area where the happiness gap increased was in educational attainment, the researchers found. People with a college diploma have gotten happier, while those with a high school education or less report lower happiness levels. Separately this month, a study found a college education remains if not a key to happiness, at least the best path to economic prosperity.