Looks like the makings of a good main post, to me. (Haven't read it all yet)

Five experiments demonstrate that people pay forward behavior in the sorts of fleeting, anonymous situations that increasingly typify people’s day-to-day interactions. These data reveal that—in contrast to the focus of media, laypeople, and prior research—true generosity is paid forward less than both greed and equality. Equality leads to equality and greed leads to greed, but true generosity results only in a return to equality—an asymmetry driven by the greater power of negative affect.