With respect to developmental continuity, it is not reasonable to expect mean level stability over the lifespan because brain growth and maturation vary by age, and thus age appropriate norms may be required. However, in longitudinal investigations, it is reasonable to expect rank order stability for an endophenotype over much of the life span. There may be age limits to stability; the P300 response of preschool-age children may not be related to psychological processes such as subjective probability in quite the same way as it does in subsequent developmental periods (Polich, Ladish, & Burns, 1990). The sensitivity of an endophenotype to changing developmental influences and the age at which it can be advantageously employed to assess genetic liability represent important aspects of construct validation. Developmental considerations aside, an endophenotype should show trait-like properties. For instance, it should show substantial test-retest reliability over short periods of time (W. G. Iacono & Lykken, 1981), and be present when clinical symptoms are inactive or during disorder remission (W. G. Iacono, Peloquin, Lumry, Valentine, & Tuason, 1982; e.g., W. G. Iacono, Tuason, & Johnson,
