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 unrelated to the effectiveness of therapy.

Second, they discovered that the type of therapy given was unrelated 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 Dawes, House of Cards: Psychology and Psychotherapy Built on Myth, New York, 1994, p. 52