Parent-child conflict interactions teach children about problem-solving and social conventions. We examined whether parents' sensitivity and the justifications they use during conflict discussions with children have unique and cumulative effects on changes in children's externalizing behaviors over time. Participants included 190 mothers and their 5- to 7-year-old children who engaged in laboratory conflict discussions at wave 1. The discussions were coded for mothers' use of conventional reasoning and sensitivity toward the child. Mothers reported on their child's externalizing behaviors at wave 1 and 1 year later at wave 2. Latent change score modeling (LCSM) was used to examine change in child externalizing behaviors, with maternal sensitivity, conventional reasoning, and the interaction between the two as predictors of the latent change score. Results showed that children with more externalizing behaviors at wave 1 tended to decline to a lesser degree than children with initially lower externalizing behaviors. Children who experienced less decline in externalizing behaviors (i.e., remained stably high) also tended to have mothers who used more conventional justifications. Additionally, the significant interaction between maternal sensitivity and conventional justifications showed that mothers who engaged in more conventional reasoning and displayed low sensitivity had children whose externalizing behaviors were less likely to decline over time. These findings suggest that reliance on obedience-focused reasoning, particularly in the absence of emotional sensitivity, may reinforce negative behavior patterns.