The measure of age at each time point was the participant’s age (X) minus the mean age at the initial assessment (17.49 years). Centering at the mean age makes our P300 intercept interpretable as the mean amplitude at this wave of assessment. This makes our intercept directly comparable to that of our past published studies reporting on P300 amplitude at this wave (Carlson et al., 2002, 2004; Carlson et al., 1999; Carlson et al., 2007; Iacono et al., 2000; Iacono et al., 2002; Iacono et al., 1999), as well as being consistent with our behavior genetic growth model using these participants (Carlson & Iacono, 2006). Although individuals were targeted for assessment at ages 17, 20, and 23, to optimize participation we allowed some flexibility in scheduling around the targeted age, especially as participants became older and more independent. A continuous measure of age rather than a dummy variable coded for assessment wave was used because of the nontrivial variability in age at some assessment points (particularly at Time 3). This also allowed for more precise estimates of growth parameters by
