We estimated the genetic similarity matrix between all individuals using the 269,616 autosomal SNPs that passed quality control and were common to at least 95% of individuals in the combined sample. We excluded one of each pair of individuals with an estimated genetic similarity of >0.05 (approximately closer than second cousins), in order to reduce the possibility that the phenotypic resemblance between close relatives could be caused by shared environmental effects and/or causal variants not correlated with SNPs but captured by pedigree (Visscher et al. 2010; Yang et al. 2010). This led to an exclusion of 4,197 individuals, resulting in a retained dataset of 8,662 individuals. To check if shared environmental effects and/or causal variants captured by pedigree were still biasing our estimate, we also tested a more stringent cut-off by excluding one of each pair of individuals with an estimated genetic relationship of >0.025 (~ closer than third or fourth cousins). This led to an exclusion of 7,957 individuals, resulting in a retained dataset of 4,902 individuals. Population structure (i.e. differences in allele frequencies between subpopulations which might also
