These quantitative approaches create other opportunities for furthering our understanding of developmental endophenotypes and psychiatric disorders. For instance, to the extent that common genes influence a developmental trajectory as a whole (i.e., intercept and slope together), deviations from expected trajectories must be due to environmental influence. This opens an avenue to studies of gene–environment interplay. By including multiple measurements longitudinal studies also permit researchers to estimate genetic factor scores (cf. Boomsma, Molenaar, & Orlebeke, 1990) on an endophenotype to be used in molecular genetic research as well as in studies of gene–environment interplay.
