ing her young, whereas in others, helpers areunlikely to breed with the dominant female,and either kin selection or mutualism proba-bly maintains cooperation (5). Similarly, theavailable evidence suggests that the relativeimportance of mutualism and kin selectionmay vary between societies.Finally, if mutualism proves to be impor-tant in maintaining cooperative animal soci-eties, the benefits of cooperation in animalsmay be more similar to those of cooperationin humans than has been previouslysupposed. In humans, unrelated individualscommonly assist each other (59), generalizedreciprocity appears to be important in main-taining many social institutions (60), and,where human groups compete, their size of-ten has an important effect on the outcome(61). “God,” as Shaw’s cynical Bastard ofOrle´ans remarks to an optimistic Joan of Arc,“is on the side of the big battalions.” All threetrends appear to have close parallels in othercooperative animals.References and Notes1. W. D. Hamilton, J. 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Carlson for her help.REVIEW: CONFLICT AND COOPERATIONCooperation and Competition Between RelativesStuart A. West,* Ido Pen,† Ashleigh S. GriffinIndividuals are predicted to behave more altruistically and less competitively towardtheir relatives, because they share a relatively high proportion of their genes (e.g.,one-half for siblings and one-eighth for cousins). Consequently, by helping a relativereproduce, an individual passes its genes to the next generation, increasing theirDarwinian fitness. This idea, termed kin selection, has been applied to a wide range ofphenomena in systems ranging from replicating molecules to humans.