This psychoterapeutic model is the result of an evolution that occurred when, as a young family therapist Richard Schwartz, began to take into account what his patients spontaneously reported to him from their experience of their inner world. They described to him what they called "parts" of themselves, sub-personalities often in conflict with each other.
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He noted that the sub-personalities (referred to as "parts" in the Internal Family Systems model) were engaged in typical roles found from one person to another.  They established within the mind the same patterns of interaction that were frequently observed between members of the same family. He found that these roles and patterns of relationships were not fixed and could evolve through prudent and respectful interventions. He began to think of the mind as an inner family and began to apply to it the techniques he employed as a family therapist. He thus created the foundations of a new form of psychotherapy.