P300 amplitude decreased with age from mid-adolescence to early adulthood. Rather than reflecting a specific vulnerability to alcohol dependence, a deviant trajectory of P300 change was associated with severe paternal externalizing psychopathology, consistent with prior findings suggesting a general vulnerability for externalizing disorder. By age 21, P300 amplitude no longer significantly distinguished the risk groups, with the low- and severe-risk trajectory groups projected to converge later in young adulthood, potentially limiting the utility of P300 amplitude as an endophenotype with older samples. Future studies are needed to confirm these findings in independent samples. Given past support indicating that the heritable influences on amplitude during this age range can be attributed to the effects of common genetic influences on the intercept and slope of amplitude growth trajectories, future research may profit by searching for genes that influence P300 change rather than cross-sectional amplitude. Our understanding of what puts people at risk for the various externalizing disorders might be advanced by use of P300 amplitude trajectory as a source of hypotheses regarding relevant neural and cognitive processes that change over the same time course.
