All participants were taken from the MTFS, a community-representative study of twins and their parents. Participants completed psychiatric assessments, self-report questionnaires and laboratory-based measures (Braff et al. 1992; Iacono et al. 1999, 2014a, 2016). The total number of individuals available with genotypes and at least one psychophysiological measure was 4905. All individuals were genotyped on the Illumina 660W-Quad, with genotyping protocols and quality control described extensively previously (Miller et al. 2012; Vrieze et al. 2014a). After quality control, the Illumina 660W-Quad contained a total of 527 829 variants.
