A new paper (well, not “new”, it’s from June, but I’m just now reading it) in Science by Samuel Bowles: Did warfare among ancestral hunter-gatherers affect the evolution of human social behaviors? (Science, 324: 1293-1298, 5 June 2009). He makes an interesting paradoxical observation, or at least hypothesis: Did altruistic behavior arise from intergroup conflict? Or rather, did cooperative behavior become a trait positively affecting fitness because of competitive behaviors between groups? It’s paradoxical because we usually think of “altruism” or “cooperation” as peaceful concepts: we cooperate instead of compete and everything is nice and friendly. What Bowles is arguing for is that perhaps competition in the form of organized warfare between groups led to selection for groups whose cooperation in the form of fighting was greater than those of other groups. Here’s the abstract:

Since Darwin, intergroup hostilities have figured prominently in explanations of the evolution of human social behavior. Yet whether ancestral humans were largely “peaceful” or “warlike” remains controversial. I ask a more precise question: If more cooperative groups were more likely to prevail in conflicts with other groups, was the level of intergroup violence sufficient to influence the evolution of human social behavior? Using a model of the evolutionary impact of between-group competition and a new data set that combines archaeological evidence on causes of death during the Late Pleistocene and early Holocene with ethnographic and historical reports on hunter-gatherer populations, I find that the estimated level of mortality in intergroup conflicts would have had substantial effects, allowing the proliferation of group-beneficial behaviors that were quite costly to the individual altruist.

And the key explanation of the mechanism he’s arguing for:

The primary behaviors thought to have been spread by war are what Darwin termed the “social and moral qualities” and other forms of altruism. This paradoxical role of war arises because, in the absence of within-group positive assortment, altruism will suffer adverse within-group selection. But it might be sustained by the between-group selection pressures that warfare introduces if altruists willingly fight on behalf of others in their group so that otherwise comparable groups with many altruists tend to prevail in intergroup contests. In game theoretic terms, defense or predation is a public good (participating is an n-person prisoner’s dilemma) in which those who participate confer benefits on their fellow group members at a cost to themselves. While I treat the case of the altruist as warrior as paradigmatic, willingness to take mortal risks as a fighter is not the only form of altruism that contributes to prevailing in intergroup contests; more altruistic and hence more cooperative groups may be more productive and sustain healthier, stronger, or more numerous members, for example, or make more effective use of information. The two key determinants of the effect of warfare on the evolution of social behaviors are the extent of genetic differences between the winners and losers of conflicts and the effect of the number of altruists in a group on group members’ average fitness. Warfare affects the second by making the presence of altruists in a group critical to the members’ survival (and hence their fitness). There are two ways in which the outcome of a conflict may affect the average fitness of its members. The first is that members of losing groups are more likely to perish, and those who die may either produce no offspring or leave children who suffer high mortality due to inadequate parental care. The second is that, as with chimpanzees (18), weaker groups cede territory, thereby redistributing fitness-relevant resources between the groups.

He acknowledges that genetic or cultural evolution may be operating (I tend to think the latter is probably a more productive avenue) and his model is based on genetic differences.

The biggest weakness from an empirical view is the archaeological samples, which he readily describes. It’s a very small subset of archaeological samples and he only includes those with skeletal evidence (burials). Not every site contains burials and sites with burials are often (especially in earlier literature) more publishable. Also, most burials are never found, not all bones from a given burial are found, and many violent causes of death do not leave traces on the bones. Nevertheless, he finds in the positive: that “the levels of warfare mortality observed in many populations would offset substantial costs of altruism.”

Actually, you can get a lot of background info here (he’s got quite a few links to papers and commentary, although some of it is sub only). I like the idea, although data limitations only make it a viable contender, not anything slam dunk. I wonder if the genetic differences are all that profound enough to actually affect behavior or if the behaviors themselves as cultural traits are a more appropriate model. In that case, you would have to consider transmission both across and within generations. Still, worthwhile taking a longer look at.