Their research concludes that groups that cooperated among themselves would have been more successful than uncooperative groups. But in order for the "cooperation signal" to be significant enough in human evolution to persist, war between groups was necessary (to sort evolutionary winners and losers). Bowles and Gintis suggest that xenophobia arises from this dynamic.
In A Cooperative Species: Human Reciprocity and Its Evolution (Princeton University Press, May 2011), Bowles and Gintis use experimental, archaeological, genetic, and ethnographic data to demonstrate how human genetic and cultural evolution has produced a species in which substantial numbers make sacrifices to uphold ethical norms and to help even total strangers.
Buy A Cooperative Species from Princeton University Press.
Read the New York Times article (August 1, 2011)
Read the PRI (Public Radio International) piece on the evolutionary roots of xenophobia (August 2011)
Bowles defines altruism in this Science News article about recent research of chimp cooperation (August 2011)