ScienceWeek

Science 26 October 2007:
Vol. 318. no. 5850, pp. 581 - 582
DOI: 10.1126/science.1150316

http://scienceweek.com/2007/071028d.htm

Evolution of Behavior: Altruism and Group Selection

Holly Arrow

Which would you prefer: a society of selfish but tolerant freetraders, or a warrior society in which people help one another but are hostile to outsiders? If you value both altruism and tolerance, neither seems ideal. Societies of tolerant altruists, however, are exceedingly rare in the simulation presented by Choi and Bowles on page 636 of this issue (1). Instead, altruism flourishes only in the company of outgroup hostility (parochialism), with war as both the engine of this coevolutionary process and its legacy. For a compatriot, the parochial altruist who risks his life is a shining knight, whereas the outsider encounters the sharp end of this altruism.

From an evolutionary perspective, altruism--acts that benefit others at a personal cost--is puzzling. Some influential theories that address this puzzle are kin altruism (2), the tendency to help blood relations; and reciprocal altruism (3), the tendency to help people who are likely to return the favor. Neither explains generosity to non-kin when costs are high and reciprocation unlikely. Heroism in warfare is an example. Explaining such extravagant altruism via indirect benefits to altruists and their kin has proved difficult. A growing body of work seeks instead to explain altruism with models that include selection on both individuals and groups.

In such "multilevel" models (4), the evolutionary outcome depends on the relative impact of competing pushes and pulls at individual and group levels. Individual selection pushes counterproductive behaviors like altruism out of the gene pool. Group selection exerts a contrary pull, favoring groups with many altruists over groups of more selfish folk. In most species, individual selection wins out. For humans living in small groups, however, a strong group selection pull is plausible. Evidence that intergroup violence killed a nontrivial proportion of our ancestors (5) has fueled interest in war as a force for robust group selection. War is a strong candidate because people kill each other based on group membership.

BOOK SOURCES:

evolution
evolutionary psychology
altruism
sociobiology