We would like to thank Dr. Clara Jones for inviting us to respond to the skepticism she expressed in the Theoretical Primatology Projects Newsletter concerning the possibility that altruism among humans might have evolved by means of group selection.1 Our view, by contrast, is that given the capacity of humans to construct institutional, cultural and other environments which reduce the force of individual selection against altruistic traits, a genetic predisposition to behave altruistically in some situations might have evolved this way. Whether this gene-culture coevolution perspective provides a sufficient explanation of the experimentally observed forms of human altruism (Fehr and Fischbacher (2003)) is very much an open question and one that should be an active focus of research.
It has been long recognized that in populations composed of groups characterized by a markedly higher level of interaction among members than with outsiders, evolutionary processes may be decomposed into between-group and within-group selection effects (Lewontin (1965), (Price (1972), Crow and Kimura (1970), Uyenoyama and Feldman (1980) ). Where the rate of replication of a trait depends on the frequency of the trait in the group and where group differences in trait frequencies are substantial and persistent, group selection contributes to the pace and direction of evolutionary change.
But most who have modeled evolutionary processes under the joint influence of group and individual selection have concluded that as an empirical matter group selection pressures cannot override individual-level selection except where special circumstances (e.g. small group size, limited migration) heighten and sustain differences between groups relative to within-group differences (Eshel (1972), Boorman and Levitt (1973), Maynard Smith (1976)).
Beginning with Darwin (for example Darwin (1873):156 and other passages), a number of evolutionary theorists have suggested that human evolution might provide an exception to this negative assessment of the force of multi-level selection. J.B.S.Haldane (1932) suggested that in population of small endogamous Atribes@, an altruistic trait might evolve because the Atribe splitting@ which occurs when successful groups reach a certain size would create a few successor groups with a very high frequency of altruists, reducing within-group variance and increasing between-group variance, a process very similar to that simulated in this paper (pp.210 ff).
William Hamilton (1975) took up Haldane's suggestion, adding that if the allocation of members to successor groups following tribe splitting was not random but was rather what he called Aassociative@ (p.137), group selection pressures would be further enhanced. Subsequently, a number of writers have pointed out that group selection may be of considerably greater importance among humans than among other animals given the advanced level of human cognitive and linguistic capabilities and consequent capacity to maintain group boundaries and to formulate general rules of behavior for large groups, and the resulting substantial influence of cultural inheritance on human behavior (Alexander (1987), Cavalli-Sforza and Feldman (1973), Boyd, Robert and Richerson (1985), Boyd, Robert and Richerson (1990), Sober and Wilson (1994), Boehm (1997) Gintis (2000)).
It is now widely accepted that the distribution of culturally transmitted traits (i.e. learned behaviors) may be strongly influenced by group selection pressures (Boyd, Robert and Richerson (2002), Soltis, Boyd, and Richerson (1995)). But many doubt the importance of group selection for traits governed by genetic transmission. Whether they are right is an empirical question: could a genetically transmitted altruistic trait evolve under the influence of group selection pressures in an environment approximating past human social and ecological interactions? The models and simulations presented in Bowles, Choi, and Hopfensitz (2003) and Bowles and Gintis (2004) suggest a positive answer. Human cultural capacities thus allow the construction of environments affecting the process of selection of both culturally and genetically transmitted traits. Gene culture coevolution thus provides another example of niche construction Bowles (2000), Odling-Smee, Laland, and Feldman (2003)
Among the distinctive human characteristics which may enhance group selection effects on genetic variation is our capacity for the suppression of within-group phenotypic differences in reproductive or material success, our patterns of social differentiation supporting positive assortation (non-random pairing), and the frequency of intergroup conflict. These observations suggest the limited applicability of the model and simulations to most other animals. However, for species in which neighboring groups including unrelated members compete for resources or in which group extinctions are common, a similar model might apply.
In these cases individually costly group-beneficial behaviors may contribute via group size or in other ways to the success of the group in avoiding extinctions or in gaining resources from neighboring groups. Examples include social mammals such as the cooperative mongoose Suricata suricatta, for which group extinction rates are inversely correlated with group size and in some years exceed half the groups under observation (Clutton-Brock, Gaynor, McIlrath, Maccoll, Kansky, Chadwick, Manser, Skinner, and Brotherton (1999)). Similarly, fire ants (Solenopsis invicta) and a large number of other ant species form breeding groups with multiple unrelated queens and practice brood raiding and other forms of hostility toward neighboring groups, with success positively related to group size (Bernasconi and Strassmann (1999)). Whether the levels of cooperation observed in these and other species might be explained in part by the causal mechanisms at work in our model is an interesting question which we have not explored.
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1 Authors’ affiliations: Bowles, Santa Fe Institute and University of Siena; Gintis, Santa Fe Institute; Fehr, University of Zurich and MIT. This note draws on material that appeared in Bowles, Choi, and Hopfensitz (2003). We would like to thank the MacArthur Foundation and the Santa Fe Institute and the Institute for Empirical Research in Economics (University of Zurich) for supporting our research.