In other words, in their view, humans first had to learn to cooperate in order to live in groups. Dr. Cloninger suggests that, as a result, people became more intelligent.

This intelligence, he says, helped early people act in a way that was good not just for themselves but also for other people.

STEVE EMBER: The desire to help others can take many forms, including charitable giving.

Aaron Graham told us what his church did on September eleventh to mark the tenth anniversary of the terrorist attacks on the United States. Most of the members are young people seeking experience by working for free in Washington. That Sunday they collected money to help people suffering from the famine in parts of Somalia.

AARON GRAHAM: "I was hoping we could raise a couple thousand dollars. But people gave so generously. They gave fifteen thousand dollars on that Sunday morning."

Charitable giving in the United States last year totaled about two hundred ninety billion dollars. That was up from two hundred eighty billion the year before. Contributions dropped in two thousand nine and two thousand eight as a result of the Great Recession.

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FAITH LAPIDUS: Giving money to a charity could be seen as one way of choosing to be a good Samaritan. But some American states have laws that require people to help in certain situations.

John Mikhail is a law professor at Georgetown University in Washington. He says there are two kinds of so-called good Samaritan laws. These are different from rules requiring teachers, for example, to report child abuse.