We examined child and family risk factors that were associated with the development of individual differences in externalizing problems across the toddler years. Our central hypothesis was that toddlers with more aggressive and disruptive older siblings would be at elevated risk for heightened levels of externalizing behavior. We tested this hypothesis in the context of other theoretically relevant risk factors: toddlers' inhibitory and internalized control, experiences of coercive parental discipline, and gender. Participants were 167 toddlers, their older siblings, and parents in a longitudinal study following younger siblings across the first 3 years of life. Mothers and fathers contributed ratings of externalizing symptoms between 1 and 18 months across the transition to siblinghood for older siblings and 18 to 36 months for younger siblings. Toddlers' inhibitory and internalized control were assessed using behavioral and parent report measures, and parents completed questionnaires concerning their use of coercive discipline. Individual differences in toddler externalizing symptoms were highly stable between 18 and 36 months of age for a sample of second-born children with older siblings, even though there was evidence of significant decline in externalizing symptoms from 18 to 36 months. As predicted, toddlers with elevated levels of externalizing symptoms tended to have older siblings with higher externalizing difficulties across the first 18 months of the younger sibling's life. Thus, these findings highlighted the need for further research on older siblings' contributions to the behavioral development of their toddler-age younger siblings. (PsycINFO Database Record (c) 2020 APA, all rights reserved).