"The systems approach to human behaviour make two general substantive assumptions: (1) The state or condition of a system, at any one point in time, is a function of the interaction between it and the environment in which it operates. (2) Change and conflict are always evident in a system. Individuals both influence their environments and are influenced by them. Processes of mutual influence generate change and development." (Longres, 1990. p. 19)

Each person in a family is part of the whole system. The whole is greater than the sum of its parts. A family can be thought of as a "holon, simultaneously a whole and a part of a larger system." (Longres, 1990, p. 266) Interaction (communication) between the parts is what brings the system to life. The study of the family must begin with the relationship and interactions each member has to each other. In systems theory, higher levels can control lower levels. The individual members are both unique individuals and part family at the same time. "The family is a bounded system in interaction with its environment. Within the family boundary are its members and their roles, norms, values, traditions, and goals, plus other elements that distinguish one family from another and the social environment ...families whose boundaries are open and flexible are the most healthy." (Longres, 1990, p. 274)

There are three subsystems in a family system: parents, parent-child, and siblings. The power structure in healthy families is hierarchical, with the parents sharing equal power and children having input in a democratic fashion. Yet, it is clear the parents are parents, and children are children. While status refers to the position and implies rank in a social hierarchy, "role refers to the more dynamic aspects of the position, it is more to do with what the people in various positions do or are expected to do...In the family, individuals only exist within the context of the roles and statuses they occupy." (Longres, 1990, p. 322)

The above is taken form an article "Systems Theory and Incest/Sexual Abuse of Children: Focus on Families and Communities", By Patricia D. McClendon. It is located at:

http://www.clinicalsocialwork.com

There is wealth of information on clinical social work at her site.