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 using the relatively greater amount of information contained in a continuous measure. Paternal risk group and offspring externalizing disorder were treated as categorical variables. The genetically mediated covariance between intercept and slope previously reported (Carlson & Iacono, 2006) makes separate interpretations of intercept and slope differences problematic. As such, we have adopted the approach of Hill et al. (1999) and used Wald tests of joint hypotheses regarding intercept and slope to test differences between groups in P300 amplitude trajectories. Such an approach tests the joint null hypotheses that groups do not differ in intercept and slope. Follow-up tests to determine age ranges in which groups differed significantly in P300 amplitude were conducted using the approach of D. J. Bauer and Curran (2005; see also Preacher, Curran, & Bauer, 2006). This approach identifies what the difference in intercept between two risk groups would be at each age and a 95% confidence interval for this value.1 We further used the approach of
