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Research Summary: Does CBT Work in the Way We Think it Does?

Updated: Jul 6

Negative thinking and processing information through a negatively biased lens (e.g., attending more to negative information than positive information) is thought to play a key role in the development and maintenance of depressive symptoms. Cognitive Behavioral Therapy (CBT) is a psychological treatment in which clinicians help patients evaluate and change their negatively biased thoughts in order to improve symptoms. Although clinicians frequently use CBT to treat depression and it is effective for many patients, it is still unclear exactly how CBT works. In addition, about one-third of depressed individuals do not improve with CBT. A better understanding of the ‘active ingredient’ of CBT may help researchers further refine this treatment to improve outcomes for patients.

In their article, ‘Cognitive change in cognitive-behavioural therapy v. pharmacotherapy for adult depression: a longitudinal mediation analysis’ published in the journal Psychological Medicine, Quigley et al. (2019) tested whether CBT actually works by first changing patients’ thoughts, which is how CBT is hypothesized to work based on the cognitive model. A comparison sample of depressed patients who received antidepressant medication was included in the study, since medication is thought to work through a biochemical mechanism rather than by affecting negative thinking. The researchers hypothesized that patients who received CBT would have a greater change in their negative thinking than patients who received medication. They also hypothesized that CBT would first reduce patients’ negative thinking, which in turn would reduce their depression symptoms, whereas this mechanism would not be observed in the medication group.

Adults diagnosed with depression were randomly assigned to receive either CBT or antidepressant medication for 16 weeks. Every four weeks throughout treatment, participants’ level of depressive symptoms and aspects of negative thinking were evaluated. The results were largely unexpected and contradicted the study hypotheses. Although patients in both the CBT and medication groups reported reduced symptoms and negative thoughts at the end of 16 weeks, patients who took medication exhibited changes in their thoughts earlier than those who received CBT. Despite this earlier change among those who took medication, by the end of the 16-week treatment, individuals who received CBT reported lower levels of negative thoughts and fewer depressive symptoms. Additionally, researchers found that changes in negative thoughts did not precede changes in depressive symptoms, which suggests that these two processes may change at the same time.

The study findings indicate that both CBT and antidepressant medication reduce negative thinking. Although patients receiving CBT had lower levels of negative thoughts at the end of treatment than patients receiving medication, this difference was small and it is unclear why changes in negative thoughts occurred earlier for patients receiving medication. The findings do not support the idea that CBT first affects negative thoughts, which then leads to improvement in depression symptoms. This is in line with previous research studies that have also examined thoughts and depression symptoms over time and have generally found that negative thoughts and depression symptoms change together rather than change in one process leading to change in the other. Quigley et al. argue that it is possible that the relationship between thoughts and symptoms may be complex and patient-specific, and so the variables should be measured more frequently to be able to better assess this relationship. The researchers hope that their results will help inform the design and methodology of future research on the mechanism of action in CBT for depression to ultimately improve the efficacy of this treatment.


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