1. Most organisms interact with a set of neighbors smaller than the deme (its trait group). Demes therefore are not only a population of individuals but also a population of groups (structured demes). 2. Trait groups vary in their composition. The minimum variance to be expected is that arising from a binomial distribution. Most populations have a higher variance than this due to (a) differential interactions with the environment and (b) the effects of reproduction inside the trait groups. 3. As a consequence of this variation, an individual on the average experiences its own "type" in a greater frequency than actually exists in the deme. Its behaviors are therefore directed differentially toward fellow types, and this is the fundamental requirement for the evolution of altruism. 4. Models are presented for warning cries and other donor-recipient relations, resource notification, the evolution of prudence in exploitation and interference competition, and the effect of differential trait-group extinction. In all cases evolution in structured demes differs from traditional individual-selection models. Individual selection corresponds to the case where there is zero variance among trait groups, that is, complete homogeneity. 5. The "threshold" variance permitting the evolution of altruism (negative fitness change to the donor) is that arising from a binomial distribution. As this is the minimum to be expected in nature, this theory predicts that at least weakly altruistic behavior should be a common occurrence (but see [9]). 6. If a population is overexploiting its resource, a decrease in feeding rate through interference may be selected for given any trait-group variation. 7. When trait groups are composed entirely of siblings (i.e., kin groups), the model is mathematically equivalent to kin selection. 8. As well as increasing population fitness, social systems may also evolve an "immunity" against group-detrimental types. 9. If a given group-advantageous effect can be accomplished through both altruistic and selfish mechanisms, the selfish mechanism will be selected. A paucity of altruistic behaviors may signify that it is usually possible to create the same result selfishly--not that altruism "cannot" be selected for