At present, it is not clear whether only a relatively small number of common genetic variants are linked to the aetiology of neurodevelopmental disorders (known as the “common disease-common variant hypothesis”, often abbreviated CD-CV) [206, 207], or if a large number of rare genetic variants is involved (“rare-variant-hypothesis” or heterogeneity hypothesis) [208]. In case of CD-CV, association analyses (e.g. GWA studies) may detect genetic variants if the studies contain enough power. It is clear that association analyses will be more difficult in rare variant cases [209].
