Until the 1920s adults demonstrated little awareness of jealousy among young children. Neither experts, nor prescriptive advice‐givers, nor parents evinced explicit concern. A rising tide of attention developed from about 1925 onward, as childrearing manuals routinely delivered dire and lengthy warnings about sibling rivalry and parents began to note the issue as one of their most pressing worries. The problem was defined both as a challenge in families with young children, where physical injury as well as lasting unpleasantness might result from uncurbed jealousy, and as a problem for the adult personality if jealousy were allowed to fester. Various strategies were recommended to minimize tension and to reassure of love; jealousy of a sibling was to be accepted but talked out, subtly labeled as childish. The reasons for this substantial shift in the rules about jealousy include changing family size, which heightened actual sibling rivalry over previous levels; expert reassessments of early childhood; and a growing desire to produce smooth, conflict‐free personalities as part of a more managerial, service‐oriented economy. While the intensity of concern about sibling rivalry abated slightly by the 1960s, the basic rules remained in place; the new tone constituted an internalization of the earlier rules and tactics. Finally, the impact of the new structures on sibling jealousy can be traced in several areas: a real change in emotional relations among siblings, as children bonded together less; new emotional tensions in family life that could alter wider parental perceptions of children; a growing desire among adults to shun jealousy as a childish emotion though often an inability fully to do so. The change in the 20th‐century pattern of jealousy deserves serious attention as part of the new complexities and repressive requirements of American emotional life.