To successfully navigate their social world, humans need to understand and map enduring relationships between people: we need a concept of social affiliation. Here I propose that our initial concept of social affiliation, available to human infants, is based on the extent to which one individual consistently takes on the goals and needs of another. This proposal grounds affiliation in a commonsense psychology that treats individuals as rational actors, as formalized in the naive utility calculus model. A concept of affiliation based on interpersonal utility adoption can account for findings from studies of infants’ reasoning about imitation and similarity, helpful and fair individuals, “ritual” behaviors, and social groups, without the need for additional innate mechanisms such as a coalitional psychology, moral sense, or general social preference for similar others. This concept of affiliation also offers a new view on the prosocial nature of imitation, placing the value of aligning with someone else’s behavior in the adoption of their goals rather than in mere similarity. I propose further tests of this concept of affiliation, and also discuss how it is likely to be relevant to reasoning and learning about social relationships across the lifespan.