To assess the proportion of phenotypic variance explained by common variants we applied LD score regression37 to results from the European ancestry meta-analysis (Online Methods). Assuming a population prevalence of 5% for ADHD39, we estimate that the liability-scale SNP heritability h2snp = 0.216 (SE = 0.014, P = 8.18×10−54; Supplementary Table 4). These estimated polygenic effects account for 88% (SE = 0.0335) of observed genome-wide inflation of the test statistics in the meta-analysis (λ = 1.200; see Supplementary Figure 11 for quantile-quantile plots); the remaining inflation, which may reflect confounding factors such as cryptic relatedness and population stratification, is significant but modest (intercept=1.0362, SE = 0.0099, P=2.27 × 10−4).
