There are several therapies that are effective in the treatment of depression, including CBT and third wave therapies (ACT is one of the types of third wave therapies), but also for example interpersonal psychotherapy, problem-solving therapy, behavioral activation and psychodynamic therapies. Studies that have directly compared different kinds of therapy usually indicate that the overall effects of these therapies are very comparable and do not significantly differ from each other. So, one could say that patients can choose the therapy that they most prefer, so if you like ACT you can choose that and it will probably be as effective as CBT. However, in depression there is much more research on CBT than on any other kind of therapy. That means that we know more about the long-term effects, combining it with antidepressants, the effects in specific target groups, like older adults, or people with general medical disorders. So, CBT definitely has the most evidence. There is no evidence that CBT is more effective, but there is more evidence for whom it works under which conditions.

I would recommend choosing cognitive behavior therapy, interpersonal psychotherapy, behavioral activation therapy or problem-solving therapy. All therapies have been found to have comparable effects in the treatment of depression, but these therapies have been examined most extensively, so you can be more certain that they actually work. In depression all therapies have been found to have comparable effects (maybe non-directive counseling is a little less effective). There is a lot of research on who benefits from which therapy, but that has not yet led to clear indications for who benefits most from which therapy. Therefore the best option is to choose your therapy based on your preference.

These are several different questions. Unfortunately it is not well-known how therapies work. It is relatively straightforward to examine IF a therapy works (that can be done with randomized trials), but it is much harder to examine HOW it works. That is not unique for psychotherapies, but is also true for other treatments and many other associations in health. We also don’t know how antidepressants work. We also know very well that smoking causes cancer, but we do not know exactly how this smoke in the lungs changes the lungs and how that leads to cancer. Some say that therapies work through a good relationship between the patient and the therapist, and other ‘common factors’ that all therapies have in common. That may be true, but there is currently insufficient evidence that that is indeed the case. It is also not known whether cognitive therapy works through changing cognitions or behavioral activation therapy through changing behavior. It is very difficult in research to examine whether change in thinking differently comes before change in depression, or the other way around. It is also possible that a third factor (for example the alliance between patient and therapist) causes both changes in thinking and changes in depression. Research on this is very complicated, because it also goes through circles and is not straightforward. The question on the consensus on what depression is: yes there is some kind of consensus in the diagnostic criteria for a major depressive episode as defined in the DSM or ICD. You need to have at least one core symptom of depression and at least 5 symptoms in total. But that is the clinical definition. Most researchers agree that depression can better be defined as a continuum, ranging from no depression to very severe depressive symptoms.

Unfortunately it is not well-known how therapies work and what the active ingredients are. It is relatively straightforward to examine IF a therapy works (that can be done with randomized trials), but it is much harder to examine HOW it works. That is not unique for psychotherapies, but is also true for other treatments and many other associations in health. We also don’t know how antidepressants work. We also know very well that smoking causes cancer, but we do not know exactly how this smoke in the lungs changes the lungs and how that leads to cancer. Some say that therapies work through a good relationship between the patient and the therapist, and other ‘common factors’ that all therapies have in common. That may be true, but there is currently insufficient evidence that that is indeed the case. It is also not known whether cognitive therapy works through changing cognitions or behavioral activation therapy through changing behavior.

Psychotherapy and antidepressant medication have comparable effects at the shorter term, but the combination of the two is clearly more effective than either one alone. That is also true at the longer term. The advantage of psychotherapy is that you get it once, while you have to keep on taking drugs. If you stop taking drugs the risk for relapse is large. Patients drop-out from therapy less often than from medication, and there are some indications that therapy alone may be more effective than medication at the longer term.

Depression gets better in many people, also when they do not get treatment. That is also the case when people are on the waitlist. It is not entirely clear why people get better ‘spontaneously’. Probably people have their own ways of coping with mood problems and some of them may succeed in solving their problems with these coping mechanisms. For example, for someone it may be very helpful to sleep regularly, start doing exercise, solve interpersonal problems, get support from people they respect. When someone is on a waiting list it may also be the case that this person is expecting to get better soon (when the treatment starts) and these expectations may also have a positive effect. But these are all hypotheses and we do not know for sure whether these are true.

There is a lot of research on the treatment format of treatments of depression and there is evidence that the format is not directly related to the outcomes. That means that therapy can be delivered individually, in groups, through the telephone, or indeed through the internet. However, internet interventions only have comparable effects as f2f interventions when there is personal support from a human being when the patient gets the internet treatment. Fully automated internet interventions probably also have effects, but they are much smaller than when a human therapist or coach is involved. One more caveat of internet interventions (also when human support is given) is that patients tend to drop out more easily from therapy then in face to face therapy. So that is something to keep in mind if you consider internet therapy.

There are more than 750 randomized trials examining the effects of psychotherapies for depression, but only a handful of studies have examined whether psychotherapy for depression has an effect on suicidality. In a meta-analysis we did a couple of years ago we found only 3 studies on this subject and they did not point at a significant effect of psychotherapies on suicidality. We did find an effect on hopelessness (and more studies), which can be seen as an indicator of suicidality, but currently there is no enough evidence to say whether or not therapies for depression have a significant effect on suicidality.

Almost all studies in therapies for depression include any patient, whether or not this patient has had a depressive episode before or not. So the best answer is that it is not known whether the treatment should be different in someone with a first episode compared to someone with a history. However, all therapies have been found to be effective, so it can also be assumed that these treatments work regardless of the history. If you have had a treatment in the past that worked well, you can certainly try again if it works, but you can also try another one. There is no evidence that any treatment works worse in this case. For people who have had several episodes in the past it is important to get a ‘relapse prevention’ treatment. There is good evidence that mindfulness-based cognitive behavior therapy and other types of cognitive behavior therapy are effective in reducing the risk for relapse.

It is important that such regulatory requirements for psychological treatments for depression are developed but also for other mental disorders. Mariana Purgato was the first author of a paper about this in Lancet Psychiatry (2020), with me and professor Corrado Barbui as co-authors. We think that these regulatory requirements are very important for the further development of the field and the implementation of therapies, also in low- and middle income countries. I think that the World health Organisation (WHO) could play a role in this.