Abstract Some simple genetic models are developed for investigating the evolution of altruism between siblings as a result of either kin selection or parental manipulation. It is shown that the critical value of the cost/benefit ratio of fitness effects, which must be exceeded if altruism is to evolve, is in general dependent both on the cost of altruistic behaviour and on the probability that an individual behaves as an altruist. For small values of the latter, the critical values of the cost/benefit ratio with kin selection approach those given by Hamilton (1972). They are always lower with parental manipulation than with kin selection. It is also shown that both kin selection and parental manipulation may lead to the establishment of an evolutionarily stable strategy, with an intermediate frequency of altruists within a sibship. For a given cost/benefit ratio which exceeds the critical value, there is usually a tendency for selection to increase to its limit of unity the amount of fitness sacrificed by the altruists, with a consequent increase in the fitness of their sibs. This may help to explain the origin of sterile castes. The relevance of these findings to the evolution of the social insects is discussed.