The prairie vole, Microtus ochrogaster forms communal groups by retention of philopatric offspring and the addition of unrelated individuals at malefemale and single female breeding units. Populations undergo high amplitude fluctuations in density with frequent low density bottle necks of 10-15 generations. We review the mating system and social behavior of the prairie vole with respect to potential for formation of genetically unique trait groups within the population as predicted by group selection theory. Hormonal-behavioral interactions reduce the opportunity for inbreeding within communal groups. Demographic traits, combined with dispersal and settlement behavior of those offspring that do disperse from the natal nest are such that breeding units would rarely include related animals. We predict that genetically distinct trait groups do not form in prairie vole populations, including during periods of low density bottlenecks.