On the Origin of Cooperation, by Elizabeth Pennisi
Science 4 September 2009: 1196-1199.
How did cooperation evolve when cheaters—those who benefit without making sacrifices—can threaten its stability? In the ninth essay in Science‘s series in honor of the Year of Darwin, Elizabeth Pennisi discusses the genetic nuts and bolts of cooperation in systems from microbes to humans.

Positive Interactions Promote Public Cooperation, by David G. Rand, Anna Dreber, Tore Ellingsen, Drew Fudenberg, and Martin A. Nowak
Science 4 September 2009: 1272-1275.
Reward is as good as punishment to promote cooperation, costs less, and increases the share out of resources up for grabs.

For those who read German, I found this on Silke Helfrich’s CommonsBlog.