It's about the simplest explanation you could expect:
Essentially, it's that every social encounter between two people involves a guess about whether or not you'll meet again in the future; you have to decide whether or not an interaction will be "one-shot" or "repeated." By modeling "one-shot discrimination" in a computer, the group has shown that it makes more sense to presume that you'll meet again down the road. … To the researchers, it suggests that "human generosity, far from being a thin veneer of cultural conditioning atop a Machiavellian core, may turn out to be a bedrock feature of human nature." Why? Because thousands of years of small-town living have left their mark.
If you're not such an evolutionary psychologist, I recommend Adam Phillips' and Barbara Taylor's lovely short book on kindness. A philosophical beach-read, if that isn't a contradiction in terms.
The Dish 