To Örebro University

ADHD is currently classified as a neurodevelopmental disorder (NDD) and clustered together with Autism Spectrum Disorder and intellectual disabilities in the recently updated diagnostic manuals (DSM-5/ICD-11). There is, however, evidence for strong genetic overlap of ADHD with not only NDDs but also with internalizing and externalizing disorders, as well as with a general psychopathology-factor. These genetic findings challenge the accuracy of the current nosology of ADHD. In this study we therefore aimed to compare the strength of phenotypic, genetic and environmental correlations of ADHD with NDDs, internalizing and externalizing disorders, to explore if ADHD is more closely linked to one of these disorder domains. We further aimed to determine the etiological overlap between ADHD and the domains after accounting for a general psychopathology-factor. We identified full and maternal half siblings in the Swedish population and linked diagnostic data from the National Patient Register. Through structural equation modeling we fitted a confirmatory factor model, where one general factor loaded onto all disorders, and disorder clusters (NDD, internalizing, externalizing) each had a factor loading onto the disorders. We allowed ADHD to have loading from each of the cluster factors and the general factor. Initial findings showed similar magnitudes of genetic overlap of ADHD with NDDs (rg= 0.52–0.70) and internalizing disorders (rg= 0.58–0.68). Further analyses will be extended to include externalizing disorders, and to estimate etiological overlap after accounting for a general psychopathology-factor. Our findings may be informative for the nosology of ADHD as well as for increasing our understanding of the etiology of psychiatric disorders.