AbstractBackgroundAn individual's overall burden of behavioural and emotional problems across childhood is associated with increased likelihood of later mental health conditions. However, the relative extent of behavioural versus emotional problems ‐ that is, the extent to which the domains are differentiated from one another ‐ may provide additional information about who is at risk of developing a mental health condition. Here, we seek to validate differentiation as an independent predictor of later mental health conditions, and to explore its aetiology.MethodsWe analysed data from ~79,000 children in the population‐based Norwegian Mother, Father, and Child Cohort Study, and linked health‐care registries. In preregistered analyses, we modelled the extent and rate of differentiation of behavioural and emotional problems between ages 1.5–5 years, and estimated associations with later symptoms (age 8) and diagnoses (after age 8). We also explored the aetiology of differentiation by estimating associations with early life exposures and, in a subset of 23,945 full siblings, assessing the impact of accounting for unobserved familial confounding.ResultsDifferentiation of behavioural and emotional problems was associated with later symptoms and diagnoses of mental health conditions, independent of total problems. Maternal at‐risk drinking (β = 0.04 [0.02, 0.06]) and parental relationship problems (β = 0.04 [0.02, 0.05]) were associated with higher behavioural relative to emotional problems at age 5. Maternal prenatal distress (|β| = 0.04 [0.03, 0.06]), concurrent distress (|β| = 0.04 [0.02, 0.06]) and parental education (|β| = 0.05 [0.04, 0.07]) predicted higher emotional relative to behavioural problems at age 5. Estimates for maternal prenatal distress and at‐risk drinking were consistent across both unadjusted and adjusted analyses accounting for unobserved familial risk.ConclusionsDifferentiation of behavioural and emotional problems in early childhood represents a valid source of inter‐individual variability linked to the later emergence of psychopathology and may be relevant for early detection and prevention strategies for mental health.