Developmental Trajectories of Psychosis and Internalising and Externalising Disorders Among Young People with Varying Levels of Child Protection Involvement: A Population-Level Linked Data Study

Despite extensive research on the risks faced by young people in out-of-home care, population-level evidence on how mental disorders develop and diverge through childhood, adolescence, and early adulthood remains limited. This study addresses this gap by modelling developmental trajectories across internalising, externalising, and psychotic domains. This retrospective population-based study examined developmental trajectories of internalising, externalising, and psychotic disorders among 414,746 young people born in Western Australia between 1993 and 2008. Using linked administrative data, group-based trajectory modelling was applied to compare young people with out-of-home care (OHC) experience, those with child protection (CP) contact but no OHC experience, and those with no CP contact. Two distinct trajectory groups were identified across all mental disorder types, based on participants with at least one recorded mental health episode. Most young people followed low stable mental disorder trajectories, while a smaller subgroup (fewer than 10% or slightly above 10% for psychotic disorders) showed elevated, increasing trajectories over time, with more pronounced escalation among those with OHC exposure, reflecting concentrated and complex developmental risk. These higher-risk trajectories reflected developmental variation in timing and severity, including earlier escalation of internalising and externalising disorders and later emergence of psychotic disorders. Factors associated with higher-risk trajectories included socio-demographic disadvantage, placement characteristics, and substance misuse. These findings indicate that mental health risk among young people with child protection involvement is not uniform but concentrated within particular model-derived developmental trajectory groups. Population-level linked data can enhance understanding of developmental mental health risk and support timelier, targeted, and integrated child protection, mental health, housing, and substance misuse responses.