Journal ArticleGroup selection may operate through selective emigration, as Sewall Wright envisioned, as well as through selective extinction. The discrete-generation model of selective emigration developed here yields the following conclusions. 1. The fitness benefit of altruism, "depends on the frequency of altruists". Consequently, selective emigration is more likely than kin selection or selective extinction to lead to polymorphic equilibria. 2. In contrast to selective extinction, selective emigration is facilitated (weakly) by high levels of mobility between groups. 3. Like selective extinction, selective emigration is facilitated (weakly) by kinstructured migration and by isolation by distance, particularly where the dimensionality of the migration pattern is low. 4. The only factor with any great effect on the strength of selective emigration is the size of the social group within which altruistic interactions occur. 5. Wright emphasized that selective emigration requires a delicate balance between the migration rate and population size, but this balance appears to be less delicate than Wright thought. For any conceivable migration pattern, migration rate, number of groups, and level of kin structure, an allele for altruism is favored only if its benefit-to-cost ratio exceeds a number of the same order as group size
