al., 1999), suggesting that the utility of this endophenotype may be limited in this developmental period. Because gene expression is itself dynamic, it is also important to consider age and development when searching for genetic variants influencing a candidate endophenotype; the relationship between gene and endophenotype may not be constant across development. The relationship between person and environment is also dynamic. On Waddington's model of the epigenetic landscape (Waddington, 1956), earlier periods of development are characterized by a flatter epigenetic landscape, permitting deviations in developmental trajectory more readily than later periods, when trajectories become increasingly canalized and constrained. Longitudinal designs may be particularly useful because they allow one to describe and characterize developmental trajectories. Empirical Bayes estimates of growth curve parameters characterizing different aspects of individual subject's developmental trajectories may well be more sensitive targets of gene discovery than are single measures, just as reducing measurement error through aggregating multiple measurements increases power. If understanding gene effects on developmental trajectories themselves is illuminating, they may also be more appropriate targets.
