age (defined as 3 dummy measures representing age at questionnaire: age less than 16 years, between 16 and 17 years and between 18 and 20 years with age greater than 21 years as the reference). The corresponding residuals, representing the RSI and PSI factor scores without these confounding effects of age, were used for all subsequent analyses including twin modeling. Compared with those aged 21 and older, those aged 14–17 years at the time of the questionnaires, had lower PSI scores (p < .0001 for those aged 14–16 years and p< .001 for those aged 16–17 years) suggesting fewer friends who used substances. All twins were 18 years and older at the time of assessment of RSI using interviews. However, younger individuals, aged 18–20 years, were considerably less likely (p < .0001) to report RSI than those aged 21 years and older.
