Answer 1: There is almost no difference in effectiveness between the different schools and kinds of psychotherapy developed so far.
Answer 2: Psychotherapy (or counselling) of any type or school is a highly effective way of helping people.
The first answer, despite many attempts to contradict it, has withstood all attacks, and is supported by an enormous amount of research evidence. It includes the difference between individual and group therapy, looking separately at children and adolescents and looking at groups of psychotherapy clients who share a similar major diagnosis such as problems with anxiety or depression.
Psychotherapy is highly effective
The second answer is as important as the first. All studies show that there is an "art and science" to psychotherapy. There is a noticeable difference between working with a psychotherapist and spending time with someone who delivers a friendly chat, or just listens to you attentively. Psychotherapy is effective, and delivers (on average).
Statistical research evidence
Both answers are statistical answers, based on tens of thousands of rigorous studies that have withstood the best scientific scrutiny. These studies involved very large numbers of people with an enormous diversity. The statistical character means that the outcomes are based on averages and probability distributions. Despite the impressive average effectiveness of psychotherapy, there remains a substantial variation around the averages, so that any individual therapy with an individual person, can do considerably better, or worse. Psychotherapy is fundamentally human and social, and people do not have the predictability and consistency of behaviour of steel or even wood.
What about the individual client?
Equivalent effectiveness does not mean that all forms of psychotherapy are equally effective for one particular person. Unfortunately no systematic ways have been developed to "allocate" clients to types of psychotherapy, say, by set of symptoms or "diagnosis". But the effectiveness of psychotherapy is enhanced if what is done during psychotherapy makes sense to you as a client, and is consistent or even appears consistent with what it is you want to work on. That is why it makes good sense when you start out with a particular problem to either meet with therapists of different background, or meet someone for a consultation (available from me and others) to discuss what form of psychotherapy would seem the best fit for you, your set of issues, and your view about what you need.
Until about the middle of the 19th century there was no psychotherapy as we now understand it. There was a general field of medical treatments, of which much was traditional and based on tradition, without evidence of effectiveness. Then there were spiritual helpers, pastors and confessors, and there was hypnosis. Especially in the USA Christian Science and the New Thought movement were influential and seen as successful. Psychoanalysis originated in Vienna, Austria, in the 1890s, and early in the 20th century got a position as the most used and most respected form of psychotherapy.
Early on there were some split offs from psychoanalysis in the 1920s and 30s, such as Adler and Sullivan. In the 1950s there was a rapid proliferation of alternative schools of psychotherapy, including first behaviourist influences and then cognitive from the 1960s onwards. With that, researchers and the public started to take an interest in comparing the effectiveness of all these different claimants to fame and results. Statistical methods kept pace with demand, and gradually allowed a clear picture to emerge.
Where to read more about it
Some time after this blog was written, on 31 March 2013, there was a good summary in different languge in this article in a business magazine.
The authoritative summary of this way of viewing psychotherapy is Michael Lambert's book with the somewhat ponderous title Bergin and Garfield's Handbook of Psychotherapy and Behavior Change. The first edition came out in 1971, and this month the sixth edition was published.
A more readable summary is to be found in the first one or two chapters of The Heart and Soul of Change - Delivering what works in therapy. (ed. by Duncan, Miller, Wampold and Hubble).
And a popular book drawing practical consequences from this way of thinking is Barry Duncan, What's Right with You.