But once you get into the details, there's a lot of debate over the happiness data. One big divide: Should you ask people how they're feeling right now, or how they feel about their life in general?

You get different answers depending on what you ask. Which one is more important is a squishy, philosophical question.

In the U.S, in order to be counted as unemployed, you have to be out of a job and looking for work. But what counts as looking for work? Checking Craigslist? Sending out three resumes a week? Five?

"It's actually kind of a hard question," says Justin Wolfers, an economist at the University of Michigan. "It's very subjective."

Yet every month, a single unemployment number is released.

But for many of us, purpose is happiness--particularly those of us who find moment-to-moment happiness a bit elusive to begin with. Martin Seligman, the positive-psychology pioneer who is, famously, not a natural optimist, has always taken the view that happiness is best defined in the ancient Greek sense: leading a productive, purposeful life. And the way we take stock of that life, in the end, isn't by how much fun we had, but what we did with it.

[...] "I think this boils down to a philosophical question, rather than a psychological one," says Gilovich. "Should you value moment-to-moment happiness more than retrospective evaluations of your life?" He says he has no answer for this, but the example he offers suggests a bias. He recalls watching TV with his children at three in the morning when they were sick. "I wouldn't have said it was too fun at the time," he says. "But now I look back on it and say, 'Ah, remember the time we used to wake up and watch cartoons?'" The very things that in the moment dampen our moods can later be sources of intense gratification, nostalgia, delight.