Carlson and Iacono (Carlson & Iacono, 2006) reported that test-retest stability correlations in male twins assessed at approximately 17, 20, and 24 years of age were quite high, ranging from .71 to .79 for three-year stability correlations and greater than .60 for the correlation across seven years. These findings indicate substantial rank-order stability in P300 amplitude despite change in mean levels. Cross-sectional heritability estimates were similar at ages 17 and 20 (.64 and .68, respectively), but dropped somewhat at age 24 (.53), indicating that at these ages, from half to two thirds of the variability in P300 amplitude could be attributed to genes. Of greater interest, however, are results of biometric regression analyses of individual growth curves in P300 amplitude. These indicated that heritability of the intercept, reflecting P300 amplitude at age 17, was much higher (h2 =.87) than heritability estimates from single (cross-sectional) measurements, and greater than the heritability of individual externalizing disorders examined in the same sample (for seven disorders, median h2 =.63, Iacono et al., 2003). Because all measurements contribute to estimates of the intercept, this study
