["Journal of Child Psychology and Psychiatry, Volume 67, Issue 5, Page 777-787, May 2026. ", "\n\nBackground\nChallenges in adaptive or daily functioning are inherent to diagnostic criteria for neurodevelopmental conditions (NDCs). However, less is known regarding the influence of factors that may confound the association between NDCs and adaptive functioning. Therefore, we examined the associations between different NDCs and adaptive functioning while adjusting for co‐occurring conditions, genetics, and shared environment.\n\n\nMethods\nWe used a co‐twin control study design in a sample of Swedish twins (N = 314, age range 8–21 years), including both monozygotic (MZ) and dizygotic (DZ) twins. Adaptive function was assessed using the parent‐rated Adaptive Behavior Assessment System, second edition. A generalized estimating equations (GEE) model was fitted, using NDC diagnoses of Autism Spectrum Disorder, Attention‐Deficit/Hyperactivity Disorder (ADHD), and Intellectual Disability (ID), as well as other psychiatric conditions, as exposure and adaptive functioning as the outcome. The model was first fitted across twin pairs and subsequently within the twin pairs, thus adjusting for genetic and shared environmental influences. Interaction effects of age and sex by different NDCs on adaptive functioning were assessed.\n\n\nResults\nAll forms of NDCs were independently associated with challenges in adaptive function across pairs. The co‐occurrence of multiple NDCs was associated with adaptive functioning, with a greater number of NDCs being associated with more functioning challenges. Higher age was associated with more challenges in adaptive functioning among autistic individuals. In the within‐pair models, the association remained for autism and ID, but the association between ADHD and adaptive functioning was lost in the MZ sub‐sample when adjusting fully for all genetic factors.\n\n\nConclusions\nNDCs are associated with challenges in adaptive function, even when adjusting for other psychiatric conditions, stressing the importance of adequate community support. Findings indicate the importance of non‐shared environmental factors for understanding the challenges in adaptive function experienced by individuals with autism and ID and genetic factors for individuals with ADHD.\n\n"]