Peer affiliation was assessed by twin and teacher reports49. Twins completed a 19-item questionnaire assessing antisocial (e.g., my friends smoke, drink alcohol, steal, get in fights; α = .85) and prosocial (e.g., my friends work hard in school, popular with other kids, liked by teachers; α = .78) peer affiliation. Teachers completed similar ratings regarding the twin's antisocial (α = .85) and prosocial (α = .87) peer affiliation (average inter-rater reliability was .71). The mean z-score of the twin and teacher reports (r = .40) was used to calculate composite measures of antisocial and prosocial peer affiliation.
