Common factors theory, a theory guiding some research in clinical psychology and counseling psychology, proposes that different approaches and evidence-based practices in psychotherapy and counseling share common factors that account for much of the effectiveness of a psychological treatment. This is in contrast to the view that the effectiveness of psychotherapy and counseling is best explained by specific or unique factors (notably, particular methods or procedures) that are suited to treatment of particular problems.

However, according to one review, "it is widely recognized that the debate between common and unique factors in psychotherapy represents a false dichotomy, and these factors must be integrated to maximize effectiveness." In other words, "therapists must engage in specific forms of therapy for common factors to have a medium through which to operate." Common factors is one route by which psychotherapy researchers have attempted to integrate psychotherapies.

Saul Rosenzweig started the conversation on common factors in an article published in 1936 that discussed some psychotherapies of his time. John Dollard and Neal E. Miller's 1950 book Personality and Psychotherapy emphasized that the psychological principles and social conditions of learning are the most important common factors. Sol Garfield (who would later go on to edit many editions of the Handbook of Psychotherapy and Behavior Change with Allen Bergin) included a 10-page discussion of common factors in his 1957 textbook Introductory Clinical Psychology.

In the same year, Carl Rogers published a paper outlining what he considered to be common factors (which he called "necessary and sufficient conditions") of successful therapeutic personality change, emphasizing the therapeutic relationship factors which would become central to the theory of person-centered therapy. He proposed the following conditions necessary for therapeutic change: psychological contact between the therapist and client, incongruence in the client, genuineness in the therapist, unconditional positive regard and empathic understanding from the therapist, and the client's perception of the therapist's unconditional positive regard and empathic understanding.

In 1961, Jerome Frank published Persuasion and Healing, a book entirely devoted to examining the common factors among psychotherapies and related healing approaches. Frank emphasized the importance of the expectation of help (a component of the placebo effect), the therapeutic relationship, a rationale or conceptual scheme that explains the given symptoms and prescribes a given ritual or procedure for resolving them, and the active participation of both patient and therapist in carrying out that ritual or procedure.

After Lester Luborsky and colleagues published a literature review of empirical studies of psychotherapy outcomes in 1975, the idea that all psychotherapies are effective became known as the Dodo bird verdict, referring to a scene from Alice's Adventures in Wonderland quoted by Rosenzweig in his 1936 article; in that scene, after the characters race and everyone wins, the Dodo bird says, "everybody has won, and all must have prizes." Luborsky's research was an attempt (and not the first attempt, nor the last one) to disprove Hans Eysenck's 1952 study on the efficacy of psychotherapy; Eysenck found that psychotherapy generally did not seem to lead to improved patient outcomes. A number of studies after 1975 presented more evidence in support of the general efficacy of psychotherapy, but the question of how common and specific factors could enhance or thwart therapy effectiveness in particular cases continued to fuel theoretical and empirical research over the following decades.

The landmark 1982 book Converging Themes in Psychotherapy gathered a number of chapters by different authors promoting common factors, including an introduction by Marvin R. Goldfried and Wendy Padawer, a reprint of Rosenzweig's 1936 article, and further chapters (some of them reprints) by John Dollard and Neal E. Miller, Franz Alexander, Jerome Frank, Arnold Lazarus, Hans Herrman Strupp, Sol Garfield, John Paul Brady, Judd Marmor, Paul L. Wachtel, Abraham Maslow, Arnold P. Goldstein, Anthony Ryle, and others. The chapter by Goldfried and Padawer distinguished between three levels of intervention in therapy:

Goldfried and Padawer argued that while therapists may talk about their theories using very different jargon, there is more commonality among skilled therapists at the (intermediate) level of principles or strategies. Goldfried and Padawer's emphasis on principles or strategies of change was an important contribution to common factors theory because they clearly showed how principles or strategies can be considered common factors (they are shared by therapists who may espouse different theories of change) and specific factors (they are manifested in particular ways within different approaches) at the same time. Around the same time, James O. Prochaska and colleagues, who were developing the transtheoretical model of change, proposed ten "processes of change" that categorized "multiple techniques, methods, and interventions traditionally associated with disparate theoretical orientations," and they stated that their processes of change corresponded to Goldfried and Padawer's level of common principles of change.