Money can't buy you love, as the song goes. But can it buy you happiness? The answer to this riddle has been the topic of extensive interest and study. The answer is yes...and no.

Daniel Gilbert, a Harvard University psychologist and author of Stumbling on Happiness, has extensively studied whether money makes people happy. He's found that when wealth lifts people out of poverty and into the middle class, it does increase happiness. After that, incremental increases in wealth do little to make people happier. Surprisingly, there's not a big difference in the happiness of the very rich and those with modest means. As our affluence increases, we begin to take former luxuries for granted and think of them, instead, as necessities.

Gilbert even puts a dollar value on the income level at which happiness peaks and levels off: $75,000. In a recent study of 450,000 Americans, participants' reported improvements in emotional well being with income increases up to $75,000. It seems once we meet our basic needs, our happiness depends more on psychological factors, such our social relationships, enjoyment at work, and sense of fulfillment.

These findings hold true on a global level as well. Although the economic well-being of the United States and many other developed countries has improved over the past several decades, residents of these countries are no happier. In fact, overall mental health of people in these societies has deteriorated as affluence increased.

There are exceptions, of course. In 2009, the Organization for Economic Cooperation and Development found that people in Denmark, Finland, and the Netherlands were the most content with their lives, even though residents in these countries pay a high percent of their income in taxes. In fact, in Denmark, workers fork over two-thirds of their income for taxes.

Other social researchers have found that absolute income does not determine happiness; a person's relative rank among his peers is a stronger predictor of happiness. In other words, we gain satisfaction when we can favorably compare our level of wealth to others, and lose satisfaction when we can't.

Raising the income of all will not necessarily increase the happiness of all. So public health experts are encouraging governments to increase access and availability to mental health care as a more effective way of improving national well being.

Despite all these findings, studies show there is one sure way that money can increase your happiness: spend it on others.