A large number of meta-analyses demonstrates that psychotherapy is beneficial for most forms of psychopathology. Yet, there is considerable disagreement as to what makes psychotherapy effective. Currently, there are two major assumptions: the specific ingredients assumption and the common-factor model. However, this dichotomy is based on conceptual inaccuracies.

The presented Taxonomy Project attempts to contribute to a clearer definition and conception of common psychotherapeutic factors by analyzing their differential associations to psychotherapeutic techniques.

A comprehensive literature search was performed to identify the common factors discussed in psychotherapy research. 22 common factors were extracted from the literature and defined. Psychotherapy experts then rated the degree of associations of different common factors to a set of psychotherapeutic standard techniques in a web-based survey. Hierarchical regression analyses using mixed models, principal component and cluster analyses were performed to analyze these associations.

Mixed-effects models indicate that common factors differ by their differential associations to specific psychotherapeutic techniques. Common factors, therefore, could be defined by their idiosyncratic profiles of associated techniques. In addition, based on these differential associations 4 superior classes of common factors operating in effective psychotherapy can be identified by principal component and cluster analyses.

At present, research addressing therapeutic change processes in psychotherapy is suffering from a terminological “jungle” and fundamental misconceptions. It is likely that this conceptual impreciseness hinders the exact analysis of the psychotherapeutic change processes and, thus, progress in psychotherapy. The presented taxonomical endeavor is a promising first step towards a clarification of the important construct ‘common factors’.