1. In order for the altruist not to be exploited by non-reciprocaters, it would be expected that reciprocal altruism can only exist in the co-presence of mechanisms to identify and punish "cheaters".

2. Students exhibiting pure altruist behavior ponied up twice as much money as their "warm glow"-only counterparts, the team reports tomorrow in Science .

3. This is because where selfishness brings higher rewards than altruism, selfish individuals leave more descendants, so altruists inevitably become extinct.

4. When addressing other Virginia planters, he spoke in the cold, hard voice of practicality, whereas when dealing with Revolutionary comrades, he blossomed into an altruist.

5. Laugh all you want, but their objectives are actually quite altruist: bringing sanitation and clean water to the 2.5 billion people around the world who still need it.