Content area
This dissertation addresses the heterogeneity in codevelopment of internalizing (INT) and externalizing (EXT) problems across childhood and adolescence. Mixed evidence and varying prevalence rates of pure and co-occurring INT-EXT patterns across time and developmental periods complicate distinctions between normative development, transient phases, and persistent psychopathology. The current dissertation examined heterogeneity in codevelopment, considering both mutually reinforcing and suppressing influences, in a large, diverse US sample drawn from the Environmental influences on Child Health Outcomes (ECHO) program. Study #1 (n = 8089) used parallel process latent class growth analysis to identify distinct joint INT-EXT trajectories from ages 1 to 18. Four co-occurring trajectory classes emerged, with most youth showing no (8%), very low (39%) or mild (38%) co-occurring problems. A smaller subgroup (15%) were likely to follow a moderate-to-high increasing trajectory class. Contrary to expectations, pure patterns were not identified. Study #2 (n = 6401) examined stability and change of INT-EXT patterns across five developmental periods using latent profile analysis and contingency-based transition probabilities. Between three and four co-occurring profiles were identified at each timepoint. As in Study #1, pure INT or EXT profiles did not emerge. Most youth had high probability to be Unaffected or Mild INT-EXT (67%–88%), with transitions typically occurring between adjacent severity levels. The prevalence of Elevated INT-EXT peaked in early adolescence. Developmentally, transitions were more common in early childhood and became more stable by adolescence. Overall, this dissertation underscores the value of developmentally sensitive, person-centered methods for capturing heterogeneity in INT-EXT codevelopment, primarily related to severity rather than direction. Findings across both studies largely aligned with transactional models, where INT and EXT problems develop in parallel across different levels of severity. However, shared vulnerability and undifferentiated distress explanations of codevelopment could also be supportive of the co-occurrence patterns. Results highlight the need for screening tools and interventions that can accommodate heterogeneous trajectories and developmental timing, with implications for early detection and prevention of persistent socioemotional difficulties.