This volume results from a symposium held at The University of Michigan, Ann Arbor, in October of 1978 and sponsored by the National Science Foundation. The papers included were, for the most part, presented at the symposium, though a few additional ones were requested fox the publication. The occasion for the symposium was the fiftieth anniversary of the University of Michigan Museum of Zoology in its present structure. It seemed to us that no better commemoration could be planned than a general discussion of the questions raised by the revolution in evolutionary biology that has occurred during the past two decades. The part of that revolution currently attracting the most attention concerns the evolutionary basis of social behavior in all parts of the animal kingdom-hence the title of the symposium and of this volume. [... ] We believe that the papers included here represent most of the topics that have sparked the recent interest in behavioral evolution. Read in sequence, the papers provide an excellent overview of current research and theory. Differences of opinion and approach are obvious and are often provocative and stimulating. We have not tried to eliminate such differences, feeling instead that each paper should stand on its own merits. We also believe that this is the first major volume of original papers devoted almost wholly to research stimulated principally by George C. Williams and William D. Hamilton, who stressed two main ideas: first, it is valuable to identify the level (gene, individual, population, species) at which natural selection acts most consistently and powerfully and, second, natural selection can favor contributions to genetic reproduction not only through descendant but also through nondescendant relatives. The importance of these two ideas is apparent throughout the volume. The organization of the volume is partly taxonomic and partly by subject. We thought it appropriate to begin with the social insects, for their sterile castes have, since Darwin, been a focal point in the understanding of natural selection. What, after all, could be more challenging to a theory of evolution based on differential reproduction than explaining the existence of individuals that normally produce no offspring of their own? The currently intensive study of cooperative breeding in birds, represented here by several investigations, involves obvious parallels, because helpers sometimes die without producing offspring; however, the conclusions reached in studies of social insects and cooperatively breeding birds often diverge intriguingly. Nevertheless, in both cases the emerging picture suggests that two crucial variables are genetic relatedness and fluctuations in the availability of breeding habitat. Nearly all of the investigators in this symposium, including those interested in caste systems and cooperative breeding, have sought to measure the reproductive success of individuals in systems of sexual competition and parental care. Data on this long neglected problem are presented for insects, fish, frogs, lizards, birds, and mammals, including humans. Sexuality can be viewed as involving a kind of proto-social cooperative behavior. Among prominent questions in evolutionary biology at present, the evolutionary raison d'etre of sexuality is fairly described as the most difficult. It is fitting, therefore, that this volume should include two papers with promising new ideas on this question. Finally, we are particularly pleased with the section on human sociality, for it shows clearly that the theory of natural selection, which has for so long guided research at all levels of inquiry in biology, has significant implications for the study of human behavior and social systems as well. [from the Introduction]