Early environmental risks and the developmental dynamics of internalizing and externalizing problems from birth to adolescence

Internalizing and externalizing problems often co-exist throughout an individual's development, shaped by a shared set of early environmental risks. However, most existing studies focused on discrete developmental periods, limiting understanding of how the negative impact of early-life adversity on mental health varies with age. Using data from 7,377 participants across seven waves of the UK Millennium Cohort Study (MCS), the current study investigated the bidirectional associations between internalizing and externalizing problems from ages 5 to 17, as well as the long-term impact of early environmental risks (i.e., prenatal influences, neonatal factors, maternal mental health, harsh parenting, and socioeconomic status) on the two symptom domains across time. Random-intercept cross-lagged panel analyses indicated that individuals with higher internalizing symptoms tended to report higher externalizing symptoms, with both symptom domains showing high stability over time. A significant positive bidirectional relationship between internalizing and externalizing problems was found from childhood to early adolescence. However, this pattern diverged in late adolescence: internalizing problems at age 14 no longer predict externalizing problems at age 17, whereas externalizing problems negatively predict subsequent internalizing symptoms. Early environmental risk factors significantly predicted both internalizing and externalizing problems from ages 5 to 14, with the effects generally decreasing with age, except for an increase at age 11. Gender differences were also observed in both the bidirectional relationships between the symptom domains and the long-term impact of early environmental risks. Findings underscore the enduring impact of early adversity on adolescent mental health and highlight the complex, evolving interaction between internalizing and externalizing problems. The study offers critical insights for early, sustained interventions that address multiple risks and adapt to adolescents' changing mental health needs over time.