Smith and Glass’s meta-analysis not only presented impressive evidence about the efficacy of psychotherapy; it concluded that three factors that most psychologists believed influenced this efficacy actually did not influence it.

First, they discovered that the therapists’ credentials—Ph.D., M.D., or no advanced degree—and experience were /un/related to the effectiveness of therapy.

Second, they discovered that the type of therapy given was /un/related to its effectiveness, with the possible exception of behavioral techniques, which seemed superior for well-circumscribed behavioral problems. They also discovered that length of therapy was unrelated to its success.

Robyn M. Dawes, House of cards: Psychology and psychotherapy built on myth, New York, 1994, p. 52