By guest blogger Tomasz Witkowski

At least one in four readers of this post will die of cancer. This is a simple statistic that leads rationally thinking people to treat the possibility as very likely. And this is what many do: they try to adopt a lifestyle that minimises the risk to some degree. But how do we know what minimises and what increases this risk? Of course, by listening to experts, the best of whom are scientists who research these things. However, whenever there is disquiet brought about by uncertainty, self-titled experts come out of the woodwork. Discussion of factors increasing the risk of cancer is today not only the domain of medical doctors and psycho-oncologists, but is also engaged in by some alternative medicine proponents, pseudopsychologists, and fringe psychotherapists, whose opinions are disseminated by journalists, some more thorough than others (see myth #26 in 50 Great Myths of Popular Psychology for more background).

Among these opinions is the common claim that negative thinking, pessimism, and stress create the conditions for the cells in our body to run amok, and for cancer to develop. Similar declarations accompany therapeutic propositions for changing our way of thinking into a more positive one that will protect us from cancer, or even cure us of the disease. Should you, therefore, begin to fear the possibility of cancer if you are not prone to optimism, or – even worse – have bouts of depression?

In a paper published recently in Psycho-Oncology, four Korean scholars have attempted to provide answers to this question by way of meta-analysis. The first difficulty they encountered was the heterogeneity of the assessment of depression in the published literature. To clarify the mixed findings, they limited data to articles that used a reliable method of defining depression based on criteria in the Diagnostic and Statistical Manual of Mental Disorders (DSM) and/or the International Classification of Disease (ICD). Only nine studies fulfilled their criteria.

What are the results? Although some low-quality studies found that patients with depressive disorder were at increased risk for cancer, overall the meta-analysis did not demonstrate that patients with clinically diagnosed depressive disorder had an increased risk of developing cancer relative to the general population.

So should readers with depressive tendencies breathe a sigh of relief? In the researchers’ cautious opinion, it is possible that some epidemiological biases affecting the diagnosis of depressive disorder or cancer could have affected their findings, thus concealing a depression-cancer link. Another limitation of their study was the narrow research database: most existing findings were obtained from Western societies and cannot be generalized to other cultures. Additionally, cancer detection depends on screening programmes and the application of new technologies, which vary across countries. Antidepressant medication is also a potential confounder for the relationship between depression and cancer. Furthermore, in the researchers’ opinion too much difference between studies causes a potential problem when interpreting the combined results. Finally, they think that possible publication bias may have affected their review, with negative results (a lack of an association between depression and cancer) less likely to see the light of day.

Scientists write articles which are most often read by other scientists. The careful wording of conclusions is, in this context, essential. However, when cautious words of authors aware of the limitations facing their work are confronted with the arrogant declarations of healers and falsehoods spread via the Internet and other media, there is reason to fear that the former will be drowned out. The meta-analysis of the Korean scholars delivers data that are in agreement with a robust body of research. Many studies have shown that such factors as positive vs. negative attitudes, mood, emotions and stress have nothing to do with cancer (for example, see here and here and here). Moreover, some of these studies demonstrated that the relationship could even be the reverse of what is expected: depressed people and those who experienced more stress were less likely to get cancer than those who were positive and did not suffer from stress.

The conviction that positive thinking and emotions prevent the development of cancer, or can even cure it, is consistent with our need for control. We prefer to live with the conviction that we have control over something rather than it being out of our hands. Unfortunately, as a matter of fact, we are powerless to influence the overwhelming majority of cancer-inducing factors, and there are still others which simply remain unknown.

—Risk of cancer among patients with depressive disorder: a meta-analysis and implications

Main image under licence from gettyimages.co.uk

Post written by Dr Tomasz Witkowski for the BPS Research Digest. Tomasz 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 https://forbiddenpsychology.wordpress.com/.