Psychopathology from childhood to adolescence in children with and without Mild Intellectual Disability (MID): a comparison across three MID operationalizations

The DSM-5(-TR) expanded the definition of Mild Intellectual Disability (MID) to include adaptive functioning (AF) alongside intelligence quotient (IQ). In this population-based study, we examined behavioural and emotional problems in children with MID and matched controls from early childhood to adolescence. We evaluated how three operationalizations of MID, varying by IQ and AF, influence outcomes. Cognitive assessments and parent-reported problems from 4,643 children from 1.5 to 15 years (T1 to T5) in the Generation R Study were used. MID was defined as: (1) IQ ≤ 75; (2) IQ ≤ 75 with impairments in AF; (3) IQ ≤ 85 with impairments in AF. Propensity score matching ensured comparable MID and non-MID groups. Linear mixed-effects models were employed to assess multiple domains of psychopathology over time. Across all MID definitions, children with MID exhibited elevated behavioural and emotional problems compared to matched controls. Children meeting the criteria for MID3, the broadest definition, showed the highest problem levels, with elevated ADHD (β = 0.37, pFDR < 0.001), affective (β = 0.25, pFDR < 0.001), and autistic problem scores (β = 0.25, pFDR < 0.001) at baseline, with differences widening over time. Children with MID1 and MID2 also displayed unfavourable ADHD trajectories, although no baseline differences were observed. The findings suggest the extent of mental health problems depends on how MID is operationalized and suggest that psychopathology may be particularly present in children with borderline intellectual disability, 75 ≤ IQ ≤ 85.