In a paper in te October 26 edition of Science by Jung-Kyoo Choi and Sam Bowles, entitled "The Coevolution of Parochial Altruism and War", they discuss the interesting dilemma why people cooperate with members of a group, but not with those outside the group. How does this conditional cooperation with non-kin evolve? Assuming conditions likely to have been experienced by late Pleistocene and early Holocene humans, nor altruism nor parochialism, will evolve on its own. Only combined altruism and parochialism evolve.
Last year, Stuart West and his colleages, published a paper in Current Biology (16(11): 1103-1106) on "Cooperation and the Scale of Competition in Humans". This paper discusses a similar phenemena as Choi and Bowles, agents are more selfish when they compete on local resources, but cooperate when agents compete for resources at a larger level. This can interpret as agents cooperate with people in a group when they compete with resources of other groups.
These papers show an interesting puzzle. Humans cooperate at high levels, also with non-kin, but also show highly competitive and selfish behavior. These studies show that there might be evolutionary indications of conditional cooperation. It would be useful if economists (Choi and Bowles) and biologists (West et al.) will cooperate with psychologists who focus on ingroup and outgroup dynamics.