It is now a common belief that a positive mood leads to better health outcomes, even (or especially) when dealing with a serious illness like cancer. Like many common beliefs, this is probably not true. (The selection process of beliefs tends to favor things we would like to be true, and not necessarily things that actually are true.) Many people have championed the efficacy of a positive mood in healing, notably Dr. Bernie Seigel, who wrote the books Love, Medicine & Miracles and Peace, Love & Healing, in which he claims:

A vigorous immune system can overcome cancer if it is not interfered with, and emotional growth toward greater self-acceptance and fulfillment helps keep the immune system strong.

But such claims are the product mostly of sloppy thinking and are not backed by the evidence. In a new large well controlled study, researchers found no benefit of positive mood in cancer survival. Study author James Coyne said of his study:

We anticipated finding that emotional well-being would predict the outcome of cancer. We exhaustively looked for it, and we concluded there is no effect for emotional well-being on cancer outcome. I think [cancer survival] is basically biological. Cancer patients shouldn’t blame themselves — too often we think if cancer were beatable, you should beat it. You can’t control your cancer. For some, this news may lead to some level of acceptance.

This study fits with prior evidence that on the whole shows very little placebo effect or benefit from mood for cancer survival. Even Seigel’s own study showed no benefit, although he later disavowed the results.

But belief in such a mood effect remains strong, and in many ways is the cornerstone of a broad range of unscientific therapies. A plausible mechanism of action is not necessary if you can simply claim mind over matter – believing in the treatment is enough. People often site the placebo effect as evidence for this mind over matter self healing.

As I have written before, the (largely misunderstood) placebo effect is not simply mind over matter, it is a complex set of effects that are mostly artifacts of observation and the failure to account for extraneous factors, not a true benefit from an inactive treatment.

One basic logical error that underlies many of the false beliefs about mood and the placebo effect is the hasty generalization, or making claims that go far beyond evidence or reason. For example, there is a consistent placebo effect in most studies with pain of about 30% – one third of people receiving only an inactive placebo will report improvement in their pain. This is often overly generalized to outcomes other than pain, but this is a mistake.

Pain is a very subjective experience and is known to be modulated by mood, attention, and expectation. Just being distracted will decrease a person’s perception of their pain. This does not mean that the mind or brain can affect all other aspects of biology also – that is can cure cancer. That is the hasty generalization.

Purveyors of many dubious health care modalities actively encourage the hasty generalization. For example, a study showing that chiropractic manipulation of acute uncomplicated back pain results in faster recovery is presented as evidence that “chiropractic works.” This one narrow indication is being generalized to all possible uses of all kinds of manipulation, including the use of chiropractic to treat asthma or ear infections. The same is true of acupuncture, where studies showing efficacy for pain (and for the record I am not convinced by the evidence that acupuncture is effective for anything) are generalized to the claim that “acupuncture works” so go ahead and use it to treat cancer. Even worse, proponents use narrow bits of evidence to claim that “alternative medicine works.” That is an outrageous generalization to a category of treatments that does not really exist – in that there is no cohesive underlying methodology or philosophy, just a hodge podge of differing (and even mutually exclusive) modalities that lie outside the bounds of scientific medicine.

In medicine all claims need to be looked at individually. Yes, there is a consilience in medicine as there is in science, so everything should make sense when considered together. And we can use our medical and biological knowledge to extrapolate cautiously and thoughtfully to new therapies (and the, of course, check to see if they actually work). But rigorous effective clinical thinking means avoiding hasty generalizations, speculating wildly without evidence, and resisting the temptation to oversimplify.

Humans in general have a strong desire to oversimplify, because it helps us deal with the complex world in which we live. Medicine, health, and sickness are very complex, and growing more complex daily as our knowledge expands. So I understand the drive to simplify, but this is not a legitimate or effective intellectual strategy. It leads to sloppy thinking and invalid conclusions.

In the final analysis, despite our desires, it seems that a positive mood does not help our bodies fight cancer – at least not significantly enough that we can measure it with existing studies. This doesn’t mean, however, that a positive mood does not help other health problems (we don’t want to hastily generalize in the other direction). It does seem to help with heart disease, for example, and this makes sense because heart function is affected significantly by our stress levels.

But what we can dispense with, based upon lack of evidence of scientific plausibility, is the magical thinking that a positive outlook can overcome all hurdles. Wishing does not make it so.