Parental monitoring was assessed as the sum of three items asked (MM3) for ages 12–14 and 15–17 based on previous work examining parental effects on risk of drug use and delinquency (Steinberg et al. 1994; Kerr & Stattin, 2000): how much parents really knew about: who the twin’s friends were, how the twin spent his money, what he did with his free time. Response options were they ‘didn’t know’, ‘knew a little’ or ‘knew a lot’. Reliability (ICC) of this variable for these two time periods was +0.75 and +0.80 respectively.
