Twin pairs for laboratory study were selected on the basis of their extreme discordance and concordance (EDAC) for self-reported scores on the Rutgers Alcohol Problem Index (RAPI; White and Labouvie, 1989) at age 18½. For a description of the realized samples for laboratory studies, including protocols not assessing ERPs and randomly selected as well as EDAC selected twins, see Latvala and colleagues (2011), where Supplementary Table 2, available online, provides summary RAPI scores for twins from discordant, concordant, and randomly selected pairs, by their zygosity. EDAC-selected twins were invited for a laboratory protocol that included a structured psychiatric interview, anthropometric measures, group and individual neuropsychological testing, a blood or saliva sample, and for the first 181 twin pairs, EEG/ERP measures. Zygosity determination of all same-sex twin pairs in the laboratory subsamples was made from multiple genetic markers assayed at laboratories of the National Public Health Institute in Helsinki.
