Happy senior man
Happy senior man. Joan Vicent Canto Roig

The belief that optimism can keep you alive—or at least stave off cancer—gained traction after the release of a study in the Lancet medical journal in 1979. The study followed six dozen recovering breast-cancer patients for five years. Researchers found that those who responded to their situation with a “fighting spirit” fared better—longer survival, fewer signs of residual cancer—than those who had feelings of “helplessness” or “hopelessness.” Subsequent studies seemed to corroborate the result, and the benefits of optimism crept into medical doctrine.

Rather pessimistically, a few recent large-scale meta-analyses (reviews of multiple studies) have found a lack of convincing evidence that optimism really extends the lives of cancer patients. Neither positive emotions like fighting spirit nor the absence of negative ones such as helplessness or hopelessness reliably predict a better outcome. “There will always be new claims, and if people look for associations, they can find them,” says James Coyne, director of the Behavioral Oncology Program at the University of Pennsylvania School of Medicine. Coyne gives one explanation for the earlier results: “If you’re healthy, and if you’re living in conditions that make you healthy, then you’ll probably be happier.”

Despite the lack of definitive data, the belief in the power of positive thinking has become so widespread that it might actually be doing harm. Cancer patients may feel inclined to act upbeat even when they’re distraught, hide their despair instead of seeking solace or treatment, or blame themselves if their disease progresses. In fact, this sort of pressure could even complicate future scientific studies of positive thinking, since it’s hard to know if a patient truly has a fighting spirit, or if she’s just pretending because she knows that’s how patients are “supposed” to act.

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This article originally appeared in the August 2013 issue of Popular Science_. See more stories from the magazine here._