Infants’ difficulty, typically characterized as proneness to negative emotionality, is commonly considered a risk for future maladaptive developmental trajectories, mostly because it often foreshadows increased parental power assertion, typically linked to future negative child outcomes. However, growing evidence of divergent developmental paths that unfold from infant difficulty has invigorated research on causes of such multifinality. Kochanska et al. (2019) proposed that parent and child Internal Working Models (IWMs) of each other are key, with the parent’s IWM of the child moderating the link between child difficulty and parental power assertion, and the child’s IWM of the parent moderating the link between power assertion and child outcomes. In Children and Parents Study (200 community mothers, fathers, and children), child difficulty was observed at 8 months, parents’ power assertion at 16 months, and children’s outcomes rated by parents at age 3. Parents’ IWMs were assessed with a mentalization measure at 8 months and children’s IWMs were coded from semi-projective narratives at age 3. The cascade from infant difficulty to maternal power assertion to negative child outcomes was present only when both the mother’s and the child’s IWMs of each other were negative. We did not support the model for father-child dyads.