The sample is broadly representative ethnically of the state of Minnesota during the relevant birth years; it is thus predominantly Caucasian (96%). To avoid population stratification, which confounds genetic analyses if allele frequencies and mean levels of a phenotype both vary by different subpopulations, we limited this study to Caucasian subjects, based on self-reported ethnicity corroborated by principal component analysis (PCA) of genotype data (Iacono et al., 2014). The mean age was 17.7 (range, 16.6–20.0) for adolescent participants and 44.6 (range, 28.4–65.3) for the parents. Fifty-seven subjects were excluded for serious head injury, neurological disorders, use of alcohol or illicit drug the day of the assessment, medication likely to affect psychophysiological responses, and not refraining from taking medications for ADHD, such as methylphenidate, as was requested of the twins (Iacono et al., 2014). We excluded an additional 126 for reasons specific to the data used in this particular report: recording problems, poor task performance (less than 75% accuracy), or insufficient data (fewer than 30 artifact-free sweeps). The final sample consisted of 4,211 individuals, 2,439 adolescents (1,180 males) and 1,772 adults (1,200 males) from 1,637 families. The majority of families were MZ twin families (1,053, or 64%).
