The family systems theory of abnormality is based on the idea that people cannot be understood in isolation from each other. Specifically within a family, equilibrium exists and the function of family roles and rules effects the individual. This equilibrium can be beneficial in family relationships in which everyone is loving and supportive of each other, but in other families, dysfunction occurs which can influence one or more family members to develop a mental disorder, which is maintained to ensure the equilibrium of the family system remains.
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Various types of dysfunctional families exist, including inflexible families, enmeshed families, disengaged families, and pathological triangular relationships. An inflexible family is one in which change is not readily accepted and any forces that promote change, either within our outside of the family, is resisted. Meanwhile, enmeshed families are so wrapped up in each other that they have no sense of privacy and have feelings of being controlled. On the other end, disengaged families are distant and don't even pay attention to one another. Lastly, pathological triangular relationships involve parents who involve their children in all aspects of the family in order to avoid conflict between themselves.
An example to illustrate family systems theory is a case in which the family is completely enmeshed and their is very little privacy or control for the children. In order to feel some sense of control, a child may begin to engage in illegal drug use activities. As the family finds out about this issue, they are likely to put further restrictions and monitoring on the child, which may drive the individual to engage in even more severe drug use. This can then create addiction and maintain a substance abuse disorder. As can be seen in this example, family relationships are often circular, and once begun, are maintained by the family structure.