This latter set of findings may be because of developmental differences in the activity of P300 generators leading to changes in the topography of P300, as suggested by L. O. Bauer and Hesselbrock (2003). They found cross-sectional evidence consistent with a shift to greater involvement of frontal P300 generators with increasing age in young people without conduct problems, but no such shift in those with conduct problems. Reduced amplitude at parietal sites might normalize with adulthood, but frontal reductions would become more apparent. Intriguingly, in a study of twins assessed longitudinally at ages 16 and 18, genetic influences on P300 amplitude in young men tended to decrease with age for posterior sites but to increase for frontal sites (van Beijsterveldt, van Baal, Molenaar, Boomsma, & de Geus, 2001). This is consistent with a genetically influenced anterior shift in P300 generators. Furthermore, although differences in topography with age are not universally supported, Mullis et al. (1985) suggested that late adolescence to early adulthood is associated with a visual P300 amplitude shift from posterior to anterior sites in a nonpsychiatric sample. The
