The second year of life is a time of formative developmental change as basic behavioral systems undergo rapid integration and expansion. This study examined the developmental trajectories of social-emotional (SoE) outcomes and the effects of infant sex and household chaos (HC) on the development of SoE outcomes across the second year of life. The participating families (N = 143) were ethnically homogenous (88% Caucasian) but economically diverse (31% low-to-very-low income). Mothers reported on their children's SoE outcomes including externalizing, internalizing, dysregulating problem behaviors as well as SoE competence when infants (54% girls) were 12, 18, and 24 months old. At each age point, HC was assessed through observations during home visits and compliance to the study protocols. Multilevel modeling revealed increasing developmental trajectories in all of the domains of SoE outcomes across the second year of life, reflecting the premise that these behavioral systems continue to form and become increasingly part of the infant's behavioral repertoire as development unfolds. However, compared to infants in less chaotic homes, infants in more chaotic households experienced steeper increases in both externalizing and internalizing problem behaviors across the second year, and girls showed higher levels of internalizing problem behaviors compared to boys. Results emphasize the increasing trajectories of problem behaviors in relation to ongoing chaotic caregiving environment among infants as young as 2 years of age.