The 12 significant loci are compelling, but only capture a tiny fraction of common variant risk for ADHD. The odds ratios for the risk increasing allele at the index SNPs in the 12 significant loci are modest, ranging from 1.077 to 1.198 (Table 1). This is within the range of effect sizes for common genetic variants that has been observed for other highly polygenic psychiatric disorders e.g. schizophrenia33. A considerably larger proportion of the heritability of ADHD can be explained by all common variants (h2snp = 0.22, SE = 0.01). This is consistent with previous estimates of h2snp for ADHD in smaller studies (h2snp: 0.1 – 0.28)23,24, and also comparable to SNP heritability estimates for schizophrenia (h2snp 0.23 – 0.26)23,24. As would be hypothesized for a psychiatric disorder, these effects are enriched in conserved regions and regions containing enhancers and promoters of expression in central nervous system tissues, consistent with previous observations in schizophrenia and bipolar disorder40. On the other hand, we do not observe substantial effects in most previously reported candidate genes for ADHD62.
