Abstract

For Sober and Wilson, the key to group selection is the persistence of within-group behaviours that are individually altruistic -- the so-called ‘primary behaviours'. In the absence of kin-relatedness, such primary behaviours will render the group liable to invasion by non-cooperative strategies unless ‘secondary behaviours’ are in place. Secondary behaviour must in effect make the costs of deviation from the primary altruistic behaviour greater than the benefits of deviation. The secondary behaviour in effect enforces the primary behaviour. Secondary behaviour is also behaviour which is altruistic from the point of view of the individual, whenever enforcement has some costs and lower benefits to the enforcer. Enforcement always has some cost, when imposed. However, it increases the rate of cooperative primary behaviour and so has benefits for the group as a whole, whence its status as altruistic. When altruism and enforcement both emerge, altruism is amplified.