There is limited evidence to support the widely held belief that psychotherapy changes the body as well as the mind

Looking at the latest epidemiological data, it could be argued that we are in the midst of a pandemic of mental illness, of dimensions never before seen in human history. The WHO estimates that over 350 million people around the world are presently suffering from depression, which constitutes almost 5-6 per cent of the population.

At its extreme, depression may lead to suicide, by which it is estimated that around 1 million people die every year. And the numbers continue growing. Faced with this rising tide of illness, it is impossible to overestimate the importance of hard facts and data indicating the paths researchers and clinicians may follow in search of ways to help.

Sometimes, as suggested by a meta-analysis of 50 years of studies on indicators that help predict suicide attempts, we are entirely helpless. In other cases, like with the recent meta-analysis of the neural correlates of the changes brought about by psychotherapy in depressed brains, study results do bring us hope. 

The results of the first systematic review and meta-analysis of biological markers evaluated in randomized trials of psychological treatments for depression in Neuroscience and Biobehavioral Reviews are another attempt at understanding methods of treating this terrifying illness. The authors – Ioana A. Cristea, Eirini Karyotaki, Steven D. Hollon, Pim Cuijpers and Claudio Gentili – quite rightly point out that understanding how psychological interventions impact or are impacted by biological variables has important implications.

For many people, their depression co-occurs with a bodily illness, such as cancer, diabetes, heart disease, and immune system and neurological disorders, and at times is a consequence of that illness. Although we still know little about the reciprocal cause-and-effect mechanisms between psychic and somatic symptoms, some studies have suggested that psychological interventions not only change mood, but also normalise the functioning of the autonomic nervous system, with a therapeutic effect on physical conditions, such as heart disease. But is this really true?

The authors analysed fifty-one trials (5,123 adult participants; over half with a somatic disorder alongside depression) where biological markers such as glycemic control, cortisol concentration after-wake, immunological, neurobiological, inflammatory, weight and blood pressure were outcomes (in 43 studies) and/or predictors of treatment response (9 studies). In fact, only one-fifth of the trials reported statistically significant physiological benefits of a psychological intervention over control, and where present, such benefits were restricted to a tiny fraction of the total variables assessed. Although statistically significant, these limited positive effects were very weak. For measures related to glycemic control and cortisol concentration, psychotherapy did not outperform the controls at all. 

The results contradict the broadly held belief that psychotherapy impacts biology. There may be many reasons for this contradiction, but a likely one is that the belief came about on the basis of purely observational evidence (the effectiveness of psychotherapy in depression is measured almost exclusively with patients' self-reports or clinician-rated scales, assessing symptoms or other subjective outcomes, and it's possible these subjective ratings have given an exaggerated impression of the biological impact of psychotherapy). 

The authors believe their failure to uncover evidence for psychotherapy having biological effects is most likely due primarily to methodological inconsistencies between studies (there was large variability across the biological domains and markers explored and overall findings were mixed), or to the confounding factors, such as placebo effects, that are so characteristic of studies on the biology of psychological illness. If they are correct, then in time, once researchers solidify their methodology, this will deliver new results that confirm the general belief in the beneficial effect of psychotherapy on disruptions to human biological functioning.

However, if the researchers' explanation is untrue, then the results of their new meta-analysis give rise to very fundamental doubts and questions: Is the dogma of psychophysical unity, presently accepted in contemporary psychology and medicine, correct? Is it possible that any associations which occur are of a solely unidirectional nature (biology modifies our mental states, but not the reverse)? And finally, perhaps the most banal question that would render the others meaningless: is it possible that the belief in psychotherapy's capacity to alter biological functions in our bodies is nothing more than the product of marketing and hype? 

In seeking the answers to these questions, it is worth examining the manner in which particular types of psychotherapy came about. Were they created and modified on the basis of analysis of feedback (such as biomarkers) supplied by patients' bodies? And if not, perhaps the issue is not with the methodological imperfections of studies looking for biological effects, but rather in the ways that psychotherapies are developed? To find answers to these questions, we have to analyse the methods that are used to shape psychotherapeutic interventions in the first place. 

Further reading

—Biological markers evaluated in randomized trials of psychological treatments for depression: a systematic review and meta-analysis

About the author

Dr Tomasz Witkowski is a psychologist and science writer who specialises in debunking pseudoscience in the field of psychology, psychotherapy and diagnosis. He has published over a dozen books, dozens of scientific papers and over 100 popular articles (some of them in Skeptical Inquirer). In 2016, his latest book Psychology Led Astray: Cargo Cult in Science and Therapy was published by BrownWalker Press. He blogs at Forbidden Psychology.