The infamous “Dodo Bird Verdict”
The Dodo Bird Verdict refers to the notion that all forms of psychotherapy are equally effective. This concept originated from a study conducted in the 1930s, which found no significant difference in outcomes between various therapy methods. A more recent study by Bruce Wampold further supported these findings, showing that “bona fide” psychotherapies (i.e., those that are meant to be therapeutic) are barely distinguishable in their effectiveness. Trials that directly compare two types of therapy rarely find differences in effectiveness. Thus, supporters of the Dodo Bird Verdict hypothesize that any variances in therapy effectiveness are not due to differences in technique, but rather to other factors such as the therapeutic relationship, the therapist's attitude towards the treatment, or the patient's expectations.
But is the case really closed on the Dodo Bird Verdict? I don’t think so. Direct comparisons of different therapeutic methods are only one piece of the puzzle. They are often biased by researchers who practice and therefore prefer one of the methods themselves. Even when researchers of different directions join forces, it is not said that the sources of bias neutralize each other - they could just as well add up. Clinical trials of psychotherapy often do not have particularly large samples, so even small biases can cause great harm. Therefore, direct comparisons should be a second step after a more important question has been answered. First, the effectiveness of each therapy method compared to control conditions such as waitlist, treatment as usual, and pill placebo, should be established. After all, the Dodo Bird Verdict states that all bona fide therapies produce equivalent effects. Without this evidence, it is hard to determine whether any differences observed between the therapies are due to their unique therapeutic components or merely due to biased data. Establishing the efficacy of each method against control conditions provides a foundation for determining which method is more effective than the others and helps researchers and clinicians determine the best course of treatment for their patients. As we will see later, there are newly developed research methods that take advantage of a large number of controlled studies to derive more valid statements about the varying effectiveness of therapeutic methods. But first, let's review what the key control conditions are in psychotherapy studies.
Finding out if therapy “works”
In general, most methods of psychotherapy show some effect when compared to control groups of patients on a waiting list. However, that is the least of the hurdles to overcome. Almost anything helps at least a little better than "nothing". Some researchers also argue that the waiting list is not merely an ineffective study condition, but can lead to negative effects. There are some control conditions that are harder to beat. For example, patients in the control condition might be given placebos that they think are antidepressants. The placebo effect is quite strong in depression and anxiety disorders, so psychotherapy has to produce quite strong effects to perform better.
Another more reliable control group commonly used in research is referred to as "treatment as usual" (TAU), or “care as usual" (CAU). TAU encompasses various control groups where patients receive some form of treatment for their condition. Typically, this means patients receive the treatment they would normally receive in their area. In certain cases, patients in the TAU group are treated by specialized physicians adhering to current guidelines. However, in other cases, TAU may involve sporadic contact with a primary care physician. Some studies are designed so that both the control and intervention groups can access TAU. This approach provides the added benefit of clearly demonstrating the effectiveness of the psychotherapy method in addition to what would be done in routine care.
State of the evidence
How do different psychotherapy methods perform when compared to these conditions and with each other? In this post, we will mainly look at studies on depression, as this is where most studies are available.
In 2021, the renowned meta-analyst Pim Cuijpers and his team published an in-depth analysis of 331 studies with 34,285 patients, comparing eight methods of psychotherapy to control conditions and the different methods with each other. They used a relatively new methodology known as "network meta-analysis". Network meta-analysis is a statistical method that allows for the comparison of multiple treatments for a specific condition by analyzing data from various studies. It can provide valuable insights into the relative effectiveness and safety of different treatment options. To evaluate and rank various treatment options based on their potential effectiveness using the “surface under the cumulative ranking” (SUCRA) method. SUCRA is a statistical approach used in clinical research to compare the effectiveness of different treatments. It ranks the treatments based on their probability of being the best option, taking into account all available evidence. They did this for several indicators of treatment success, but I will focus on the strictest criterion here: Remission, that is, the achievement of a state comparable to non-depressed individuals.
Based on SUCRA, the method most likely to be superior to all other methods was behavioral activation, a method that aims to actively engage patients in behaviors that promote positive emotions and improve overall well-being. A close second is “problem-solving therapy”, a cognitive–behavioral intervention geared to improve an individual's ability to cope with stressful life experiences. Like all other included therapies, these two methods were significantly more effective than waiting lists and TAU. But they were the only ones to be more effective than placebo pills when it comes to achieving remission. Good old cognitive behavioral therapy and its “third wave” varieties that include mindfulness or other extensions come next. They are closely followed by Interpersonal Therapy, an approach that focuses on improving communication and relationships with others. Psychodynamic therapy and non-directive supportive counseling were less likely to be good first-line choices. This does not necessarily mean that they do not work at all, or that the differences to the high-ranking methods are large, but they are less likely to be the most effective option compared to other available treatments.
In the following table, I summarize the ranking by showing the three methods most likely to be the best option for treating depression. It also includes the outcome criteria “response” (>50% reduction of depression symptoms) and SMD (difference in depression symptoms compared to the control condition).
Summary of the findings of Cuijpers et al. (2021). Numbers indicate ranks, i.e., first, second or third most effective. Rem = Remission, Resp = Response, SMD = Standardized Mean Difference. CBT = Cognitive-Behavioral Therapy, BA = Behavioral Activation, PST = Problem Solving Therapy, TWT = Third-Wave Therapies
Readers familiar with cognitive behavioral therapy methods will note that all the highly ranked methods are variations of CBT. All of them can be provided by cognitive-behavioral therapists with little to no additional training. For a patient presenting with depressive problems, the methods might be delivered one after the other. I recently posted results from a similar study that ranked treatments for post-traumatic stress disorder. Based on these findings, some methods may be better choices:
Is the Dodo dead?
Proponents of the Dodo Bird rejoice: even the Cuijpers study did not show differences between therapy methods for treating depression. What it did show, however, is differences in evidence for these methods. Some of them were more consistent in being the most effective choice in the available studies and only few of them beat all of the three control conditions. I would therefore view theoretical approaches that postulate the same effectiveness of all methods in principle with skepticism. Instead of focusing on direct comparisons, all the available evidence should be considered when evaluating treatment effectiveness. Finally, the Dodo Bird Verdict is driven by one type of disorder: depression. There are much more studies on psychotherapy for depression than for other disorders and things might be vastly different for, for example, panic disorder. Thus, the Dodo Bird Verdict must be evaluated separately for every type of problem.