Risk factors for complex disorders tend to be of small effect size (odds ratios of <2), and this has certainly proved to be the case for individual SNPs. Conversely, the types of CNVs that have been found to be associated with neurodevelopmental and psychiatric disorders have been of considerably greater effect size.2, 3 This is not surprising, because as the types of CNV mutations that have been studied are so rare, their effects have to be large to be detected in modest sample sizes. In fact, so far large rare CNVs show the strongest and most consistent associations, and size and burden of CNVs seem to be proportional to phenotypic severity. Large rare CNVs appear to be further enriched in individuals who have ASD or ADHD with comorbid ID,3, 4 and the burden appears to be even greater in those with ID and congenital anomalies.2, 3
