This measure was based on factor analysis of 9 questionnaire items where the twins reported on their peers’ substance use behaviors yielded a factor score representing PSI. Twins were asked to endorse ‘how many of your friends would you guess’: (a) smoked cigarettes regularly, (b) used smokeless tobacco, (c) drank alcoholic beverages regularly, (d) got drunk at least once a week, (e) smoked marijuana or hashish, (f) used crack cocaine, (g) used cocaine in powder form, (h) used heroin or (i) sniffed glue, gases or spray. Responses were provided on a 5-point Likert scale ranging from ‘None’ to ‘All’. The pattern of endorsement of the PSI items is shown in Table 1.
