In sociology, the concept "group" implies more than simply an aggregate of individuals. Additional elements involved are (1) structure—interaction patterned in terms of statuses and roles, (2) history—some frequency and regularity of interaction over time, (3) interdependence—some degree of members' mutual reliance on each other for needed or valued material and nonmaterial resources, and (4) common identity—grounded in shared meanings, values, experiences, and goals. Frequently there is a group product, not necessarily of a material nature, which is the outcome or consequence of collective effort and interaction.

These elements are dimensional in that groups possess and manifest them to a greater or lesser degree. At one extreme, family groups typically have well-established and enduring structures, share extensive histories, encompass a wide range of activities, exert a broad scope of influence, and provide the basis of individual identity. At the other extreme, ad hoc work groups (and...