P300 amplitude reductions in those with early-onset substance problems appear to preexist the development of heavy substance involvement or other externalizing psychopathology (Carlson et al., 2007) rather than reflect a potentially more severe effect of substance use at younger ages influencing neurodevelopment. A study of genetic and environmental influences on P300 amplitude development in twin pairs from the same sample reported on herein (Carlson & Iacono, 2006) found that 87% of the variability in the stable variance associated with intercept was because of individual differences in genetic effects. All of the correlations between slope and intercept appeared to be because of a common set of genes influencing both growth parameters. Furthermore, all of the heritable influence on P300 amplitude measured at each age could be attributed to genetic effects that were shared across age. As the relationship between externalizing and P300 amplitude in adolescence appears to be entirely genetically mediated (Hicks et al., 2007) and all of the genetic influence on amplitude in this age range is mediated by intercept and slope, it is likely that the reduced intercept of
