Within the discipline of biology, General Systems Theory (GST) has spanned almost a century. Internal debates continue to center on “living” versus “non-living” systems, entropy versus negentropy, and unit of analysis (which may mask entropy). The Gaia Hypothesis takes the debate to the planetary level.

In the family field, systems theories have multiplied and flourished. Therapists tend to rely on the style of theory adapted by the founders of MRI in the 1950s, which attempted to explain disorders such as schizophrenia. Additionally, therapists and counselors may utilize Bowen's family systems theory.

Other child and family professionals are more familiar with the Bronfenbrenner model which is essentially a biological systems model. In other fields, book authors have introduced systems concepts without making mention of earlier works. Thus, in some arenas, practitioners assume that their field invented systems. The common base is von Bertalanffy's classic work from the 1920s and expansion of the theory during World War II. Systems thinking flourished in the family field from the 1970s onward.

Concepts of the theory (variations exist):

• The whole is larger than the sum of its parts.
• Sub-systems operate within a larger system and they are hierarchical.
• Supra-systems operate outside the system.
• Systems may be characterized as open or closed.
• Boundaries within or between systems are described variously by scholars: diffuse, permeable, open, closed, rigid, flexible, ambiguous, unambiguous.
• Interaction characterizes the parts of a system and interactions are nonlinear.
• Feedback loops may be positive (promoting change) or negative (retarding change).
• Equilibrium and homeostasis are central to systems processes.

Criticisms of the theory (variations exist):

• Systems is an approach, not a theory.
• It is too abstract.
• It is too general.
• It supports conceptualization of events but offers no real explanatory power.
• Language of the theory trivializes women's experiences.
• It does not acknowledge the power differentials that exist between parts of the system.
• It assumes that a system's parts contribute equally to dysfunction and thus does not responsibly address partner or family violence.

Search terms for follow-up investigation: cybernetics, MRI, information theory, Gaia Hypothesis