We also assessed multiple domains of each twin's environmental context including: academic achievement and engagement, peer affiliation, parent-child relationships, and stressful life events. Academic achievement and engagement was a composite of twin and mother reports regarding the twin's cumulative grade point average (GPA); self and maternal ratings regarding their expectation of the twin's ultimate educational attainment (e.g., complete high school, complete college, etc.); and a 7-item scale completed by the twin and mother assessing the twin's attitudes and engagement in school (e.g., good attitude about school, enjoys attending school; α = .83)47. The measure of academic achievement and engagement used in the analyses was the mean z-score for ratings of GPA, academic expectations, and academic attitudes averaged across the twin and mother reports (r = .77). While academic achievement and engagement can also be conceptualized as an outcome or individual differences variable, in the current investigation, the construct serves as an indicator of the individual's broader environmental context (in particular the context actively or evocatively shaped by the individual)48 that might amplify or suppress risk for EXT.
