Patterson's coercion model describes a gradual escalation in maladaptive parent-child transactions whereby externalizing behaviors lead to increases in maladaptive parenting and vice versa. The current study investigates the role of (predominantly mother-reported) harsh parenting practices in the within-person development of conduct problems, hyperactive/inattentive behaviors, and emotional problems. A random-intercept cross-lagged panel model was fit across ages 3, 5, and 7 (N = 14,037, 49% female, 84% White) using the UK population-representative Millennium Cohort Study. Findings support Patterson's coercion model, providing evidence for reciprocal within-family relations between parenting practices and child behaviors. They suggest the importance of addressing parenting difficulties in families where children present with socioemotional difficulties in order to help prevent the accumulation of additional issues.