The dramatic changes in behavior that occur over the developmental stages of infancy reflect the maturation of brain circuits which sub serve more complex functions. The invariant order of both the ontogenetic maturation of brain structures and the connections between these structures into functional circuits reflects the activation of timed gene action systems that encode a timetable of emergent capacities (R. S. Wilson, 1983). Genetic systems responsible for more complex structure-function relations are sequentially activated in postnatal development. These hereditary expressions require transactions with the environment (Plomin, 1983), and in fact different and specific gene-environment interactions occur over the course of development (Kendler & Eaves, 1986). What biological mechanism could mediate the postnatal environmental activation and repression of specific neurogenetic programs?