--- - |2 Abstract Psychologists have long tried to understand why trajectories of socialization in individual parent–child dyads can be distinct, leading to adaptive or maladaptive developmental outcomes. In this article, we elucidate origins of those differences by examining the subtle yet enduring implications of early parent–child relationships in longitudinal studies of low‐ and high‐risk families, using correlational and experimental designs, and multiple measures. Those relationships are key for socialization because they can alter cascades from children's biologically based difficult temperament to parents' negative control to negative children's outcomes, as demonstrated by social‐learning theories. We suggest that those cascades unfold only in parent–child dyads whose early relationships lack positive mutuality and security. Such relationships set the tone for adversarial cascades. In contrast, early mutually positive, secure relationships initiate cooperative, effective socialization and defuse risks of negative cascades. Parents' and children's internal representations of each other may explain how such divergent sequelae unfold. - Child Development Perspectives, EarlyView.