Evolution of Cooperation, by Nowak

It's long, so some choice quotes:

"So you've got to always cooperate by random drift. But if we'd always cooperate, now we can guess what happens next: we invite Always defect. You have oscillations here, you have a mathematical model of human history, of economic cycles, of ups and downs."

"There is one thing that I have learned in my studies of cooperation over the last 20 years: there is no equilibrium. There is never a stable equilibrium."

"In an experimental study, people go up and down in trains in Britain and listen to what people are talking about, to find that about 60 percent of the conversation topics fit into this general framework of indirect reciprocity."

"What is very important for efficient indirect reciprocity is language.  Indirect reciprocity leads to the evolution of social intelligence and human language.  In order to evaluate the situation, you have to understand who does what to whom and why.  And we have to have a way to talk about what happened, to gain experience from others."

"As my friend David Haig at Harvard put it very nicely, "For direct reciprocity you need a face; for Indirect reciprocity you need a name."

"If too many others imitate it, it starts to become a losing strategy."