That conclusion comes from a new review of more than two dozen studies looking at the question of how, if at all, mental state affects the mortality of cancer patients and the recurrence of their disease.
While the results may seem disappointing, experts say some people may feel liberated knowing they don't have to force a happy face throughout their illness.
"Cancer patients feel pressure to always have a positive outlook, to put on a particular front," says Mark Petticrew, a psychologist at the MRC Social and Public Health Sciences Unit of the University of Glasgow, who helped conduct the research. "There may be good reasons for doing that, but it's not going to affect survival."
Neither will a negative attitude, the review also found.
Karolynn Siegel, director of the Center for the Psychosocial Study of Health and Illness at Columbia University's Mailman School of Public Health in New York City, says the study results weren't surprising.
"I would have been surprised if you did see the effect," Siegel says.
The notion that cancer patients could improve their outcomes through a sunny outlook gained momentum from a 1989 study that suggested women with advanced breast tumors lived longer if they attended support groups.
The finding, which has never been replicated, sparked a surge of interest in support groups, Siegel says.
Petticrew and his colleagues, whose findings are reported in this week's British Medical Journal, reviewed 26 studies of emotional coping and cancer survival or recurrence conducted throughout the world. The cancer types included leukemia, head and neck tumors, breast, colorectal and pancreatic disease. Psychological measures involved "fighting spirit," helplessness and hopelessness, denial and avoidance.
Some studies seemed to show a positive attitude had an effect on survival. However, after accounting for methodological flaws, the evidence was unconvincing, Petticrew says.
Similarly, the researchers found no solid evidence that a pessimistic perspective hastened death or the return of cancer.
"There's no consistent picture at all," Petticrew says.
Still, there are other reasons to be optimistic. A positive outlook can improve patients' quality of life, reduce their levels of anxiety and depression, and may even help them comply with their medication, Petticrew says.
Siegel adds that a cancer patient once told her, "'There's more to living than not dying.'"
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