BackgroundThere has been a limited understanding of the longitudinal trajectory and determinants of socio-emotional outcomes among children in out-of-home care (OOHC). ObjectivesThis study aimed to examine child socio-demographics, pre-care maltreatment, placement, and caregiver factors associated with trajectories of socio-emotional difficulties of children in OOHC. Participants and settingThe study sample (n = 345) included data from the Pathways of Care Longitudinal Study (POCLS), a prospective longitudinal cohort of children aged 3–17 years who entered the OOHC system in New South Wales (NSW) Australia, between 2010 and 2011. MethodsGroup-based trajectory models were used to identify distinct socio-emotional trajectory groups based on the Child Behaviour Check List (CBCL) Total Problem T-scores completed at all four Waves 1–4. Modified Poisson regression analysis was conducted to assess the association (risk ratios) of socio-emotional trajectory group membership with pre-care maltreatment, placement, and caregiver-related factors. ResultsThree trajectories of socio-emotional development were identified: ‘persistently low difficulties’ (average CBCL T-score changed from 40 to 38 over time), normal (average CBCL T-score changed from 52 to 55 over time), and clinical (average CBCL T-score remained at 68 over time) trajectories. Each trajectory presented a stable trend over time. Relative/kinship care, as compared with foster care, was associated with the “persistently low” socio-emotional trajectory. Being male, exposure to ≥8 pre-care substantiated risk of significant harm (ROSH) reports, placement changes, and caregiver's psychological distress (more than two-fold increased risk) were associated with the clinical socio-emotional trajectory. ConclusionsEarly intervention to ensure children have a nurturing care environment and psychological support to caregivers are vital for positive socio-emotional development over time among children in long-term OOHC.