Participants were twins initially recruited through the Australian Twin Registry, and other members of their families. Individuals were selected from (a) a large-sibship study (BIGSIB) designed to study families with 5 or more offspring sharing both biological parents and unselected for phenotype (Hansell et al., 2008; Saccone et al., 2007); (b) an alcohol extreme discordant and concordant (EDAC) study (Hansell et al., 2008), designed to focus on families extremely discordant or concordant for heavy drinking and alcohol dependence risk; and (c) the Nicotine Addition Genetics (NAG) study, which targeted families based on heavy smoking index cases with one or more full sibling smokers identified in previous interview and questionnaire surveys (Loukola et al., 2008; Pergadia et al., 2009; Saccone et al., 2007). Self-reported ancestry of the participants is predominantly Anglo-Celtic and Northern European (>90%) with information available on the birthplace and ethnicity of their four grandparents. Eighty percent (N=1906 unrelated individuals) of the pooled GWAS sample provided self-report on ancestry and birthplace of all four grandparents. Of these, 95.6% had all European grandparents (with 70% of these Northern European, N=1281),
