A major endeavor in modern developmental psychobiology is to elucidate the origins of novel adaptive capacities in the individual and the species. On the premise that epigenesis proceeds through progressive differentiation and functional integration, the goal is to uncover mechanisms common to both the attainment of normative endpoints within the species and the occurrence of deviations from these universals during development (Gottlieb, 1992; Kuo, 1976; Lehrman, 1953; Schneirla, 1966). Because this question is often regarded as the central concern of developmental research, it is rarely distinguished from another one requiring us to explain how psychological and behavioral capacities, once established, support malleable and reversible adaptations during ontogeny. An analysis of functional integration from the standpoint of interactive processes may be sufficient to explain how development reaches specific endpoints. But the second problem, the issue of continuity and change, requires in addition an analysis of how the organism establishes, maintains, and reorganizes its relationship with the environment over ontogeny (Cairns, 1979; Piaget, 1967).

With a specific reference to social behaviors, Magnusson and Cairns (this volume) propose that these actions “have distinctive properties in adaptation because they organize the space between the organism and the environment, and thereby promote rapid, selective, and novel adaptations.” More broadly, they suggest that behavior plays an integrative role at the interface of intra-and extraorganismic activity. While recognizing the interdependence of activity within the organism and activity within the organism's natural ecology, their perspective also postulates a necessary boundary between the two domains of activity.