We can extend the notion of association to brain measures: a genetic variant is said to be associated with a brain measure if it helps to predict thatmeasure (however weakly), using standard linear regression. However, genome-wide association studies (GWAS) are exclusively focused on a subset of common genetic variants, which do not represent the entirety of total genetic variation. As a result, GWAS rely on the linkage disequilibrium (statistical correlation) among nearby genetic variants, which arises naturally and depends to some extent on population ancestry.