Early-childhood temperament deviations mark psychiatric risk into early adulthood.

Early-childhood temperament is associated with mental health outcomes decades later. Temperament reflects early-emerging individual differences in emotional and behavioral tendencies. These differences are relatively stable across development and shaped by both genetic and environmental influences. However, the consequences of departures from expected developmental trajectories remain largely unexplored. Using data from more than 50,000 children in the Norwegian Mother, Father and Child Cohort Study, we modeled longitudinal temperament trajectories at 1.5, 3, and 5 years of age and quantified deviations from expected development. Multivariate pattern analysis revealed latent dimensions linking these deviations to clinical diagnoses, with ADHD as the most prominent outcome. Time-to-event analysis showed that these dimensions were associated with a higher hazard of ADHD diagnosis across childhood and adolescence. Finally, genetic analyses identified loci jointly associated with temperament trajectories and ADHD, revealing age-dependent genetic effects. Together, these findings show that deviations from temperament trajectories in early childhood capture transdiagnostic vulnerability across development. Early temperament monitoring may thus serve as an indicator of later mental health risk.