A systematic understanding of the aetiology of neurodevelopmental disorders (NDDs), their co-occurrence, and co-occurrence with other conditions during childhood and adolescence remains incomplete. The current meta-analysis bridges gaps in our knowledge. First, we reviewed the literature on the relative contribution of genetic factors to NDDs. Second, we meta-analysed the literature on the genetic overlap between different NDD categories. Lastly, we synthesized the literature on the co-occurrence between NDDs and disruptive, impulse control and conduct disorders (DICCs). We performed multilevel, random-effects meta-analyses on 331 independent studies comprising over 4 million individuals. We found all NDDs to be substantially heritable (grand family-based heritability h2= 0.66; grand SNP h2= 0.19). Meta-analytic genetic correlations between NDDs were moderate. Genetic underpinnings of co-occurrence between NDDs and DICCs were strong, but given the paucity of available studies, could only be estimated for a small number of disorders. While our work provides direct evidence to inform and potentially guide clinical and educational diagnostic procedures and practice, it also highlights the imbalance in the research effort that has characterized developmental genetics research.
