Still, people appear to be extraordinarily altruistic, and not just within their own families. Americans in particular are famously generous, donating about $300 billion a year to charity, more than 2 percent of the nation's GDP. Just think back to the last hurricane or earthquake that killed a lot of people, and recall how Good Samaritans rushed forward with their money and time.

But why?

Economists have traditionally assumed that the typical person makes rational decisions in line with his own self- interest. So why should this rational fellow -- Homo economicus, he is usually called -- give away some of his hard- earned cash to someone he doesn't know in a place he can't pronounce in return for nothing more than a warm, fuzzy glow?

Building on Gary Becker's work, a new generation of economists decided it was time to understand altruism in the world at large. But how? How can we know whether an act is altruistic or self- serving? If you help rebuild a neighbor's barn, is it because you're a moral person or because you know your own barn might burn down someday? When a donor gives millions to his alma mater, is it because he cares about the pursuit of knowledge or because he gets his name plastered on the football stadium?

Sorting out such things in the real world is extremely hard. While it is easy to observe actions, it is much harder to understand the intentions behind an action.

Laboratory experiments are of course a pillar of the physical sciences and have been since Galileo Galilei rolled a bronze ball down a length of wooden molding to test his theory of acceleration. Economists, however, have never been as reliant on the lab. Most of the problems they traditionally worry about -- the effect of tax increases, for instance, or the causes of inflation -- are difficult to capture there. But over time, some economists came to believe that if the lab could unravel the scientific mysteries of the universe, surely it could help figure out something as benign as altruism.

These new experiments typically took the form of a game, run by college professors and played by their students. One was called Dictator. In this game, a small pool of money is divided between two people, but only one of them gets to decide how the money is divided. (Thus the name: the "dictator" is the only player who matters.)

The original Dictator experiment went like this. Annika was given

$20 and told she could split the money with some anonymous Zelda in one of two ways: (1) right down the middle, with each person getting $10; or (2) with Annika keeping $18 and giving Zelda just $2.