how many PCs were appropriate to control for ancestry stratification effects within their specific sample. Because S4S participants were recruited at university, parental rather than own education level was included as a covariate in this sample. In 25Up, S4S, and NTR we used 10 PCs to control for population stratification, while in UKB we included 40 PCs. We controlled for clustering due to genetic relatedness in the twin datasets (25Up and NTR) by using the family option in PLINK and excluded individuals that showed high genetic relatedness in the other datasets (see Table S1).
