The origin of cooperation is one of the major transitions in biology, is intimately tied to family-living and the evolution of complex animal societies including humans, and helps explain the impacts of certain pests like termites. Why do some kinds of animals form complex societies, while other closely related ones do not? Are complex insect societies similar to those of vertebrates? Although researchers have long recognized similarities between social vertebrates and invertebrates, the several conspicuous differences among social animal groups have historically divided the field of social biology, leading to distinct terminologies, approaches, and theoretical frameworks. Few studies have systematically examined how explanations for sociality in vertebrates or insects are related, or whether they apply to other family-living species. One promising approach to resolving these issues involves testing different modes of social evolution in a single group of related species that exhibits a range of social systems. This comparative study will evaluate the evolution of family-living within a socially diverse group that unusually exhibits traits of both social vertebrates and insects, the sponge-dwelling snapping shrimp, Synalpheus. Using molecular methods, this research will focus on how the cohesive (kinship) and disruptive (conflict) influences of group-living interact to mold social organization and how these interactions vary among closely related species, which may serve as proxies for stages in the early evolution of family-living. The results will illuminate several key problems, including the evolution of altruism and cooperation, the roles of family structure and conflict in the evolution of sociality, transitions among social systems, and links between social evolution and the diversification of morphology and behavior. Social shrimp have demonstrated high value in public education and outreach, and findings on the evolution of family-living in these charismatic marine animals will be disseminated to K-12 schools and the general public via an award-winning website and school lectures.