Statistical analyses to estimate the proportion of variation explained by all SNPs were performed as in Yang et al.28. This method captures the variance in the trait that is due to linkage disequilibrium between genotyped SNPs and unknown causal variants28,30. The interpretation of the estimate of variance accounted for is different from that estimated from traditional family or twin studies, because these latter designs capture the variation due to all causal variants in the genome and, possibly, variation due to environmental factors, for example when the effect of shared environment is larger in monozygotic twin pairs than in dizygotic pairs 30. We firstly estimated pairwise genetic relationships between 3511 individuals from CAGES from 549 692 autosomal SNPs. We excluded one individual from any pair that had an estimated coefficient of relatedness of > 0.025 and retained 3291 individuals for analysis: 1800 from Scotland and 1491 from Northern England. The reason for excluding close relatives is that: (i) their phenotypic covariance will have a large impact on the estimate of variance explained; (ii) the phenotypic covariance of close relatives captures the
