We propose a model linking the early parent-child mutually responsive orientation (MRO), children's temperament trait of effortful control, and their internalization of conduct rules. In a developmental chain, effortful control was posited as a mediator of the links between MRO and children's internalization. MRO was further posited as a moderator of the links between effortful control and internalization (i.e., moderated mediation): Variations in effortful control were expected to be more consequential for internalization in suboptimal relationships, with low MRO, than in optimal ones, with high MRO. The model was tested in 2 studies that employed comparable observational measures. In Family Study (N = 102 community mothers, fathers, and children), MRO was assessed at 25 months, effortful control at 38 months, and children's internalization at 67 months. In Play Study (N = 186 low-income, diverse mothers and children), MRO was assessed at 30 months, effortful control at 33 months, and children's internalization at 40 months. MRO was observed in lengthy naturalistic interactions, effortful control in standardized tasks, and internalized, rule-compatible conduct in parent-child interactions and in standardized paradigms without surveillance. Structural equation modeling analyses, with internalized, rule-compatible conduct modeled as a latent variable, supported moderated mediation across mother- and father-child relationships and both studies. In optimal, mutually responsive relationships, multiple mechanisms other than capacity for effortful control may also operate effectively to promote internalization, thus reducing the relative importance of variations in child temperament.