Friday the Thirteenth is a fitting time to remind ourselves that there's no evidence risks and probabilities in the external universe the brute, physical facts of reality change in response to human attitudes. Your optimism or pessimism may alter your own behavior, but it doesn't change the odds of you winning the lottery, getting hit by a meteor, or having your picnic rained upon (any more than the number thirteen does). It will rain or it will not rain, and the odds are in no way tied to whether you remembered your umbrella, despite the common belief in "fate."
Similarly, though less obviously, there is no good evidence that prayer or a positive outlook can arrest the progress of disease (though both might calm a person during pain or be correlated with changes in behavior that have a more direct physical impact on one's likelihood of survival, such as a greater willingness to stick to a medical regimen). This point was reinforced by two recent reports: a study by British researchers showing that a positive outlook does not affect the progress of cancer (despite frequent claims to the contrary by alternative medicine aficionados) and a Wired.com article raising serious questions about a study widely reported to show that people who are prayed for even ones who do not know they are being prayed for are more likely to heal.
When the study first appeared, reporters eagerly spread the word about it, and few asked probing questions at the time about the objectivity of the researcher, Dr. Elisabeth Targ. From Wired.com, we learn that, far from being a paragon of scientific impartiality, she had literally been raised since birth to believe in psychic phenomena by a father who himself did research in psychic phenomena for U.S. military intelligence and who had subtly scolded his daughter when she was a child if she was unable to psychically identify presents before they were unwrapped and removed from their boxes.
His daughter grew up to surround herself with professed psychics and energy healers, many of whom looked to her for potential vindication of their views and tried to heal her with their imagined powers when she became terminally ill with a brain tumor herself, around the time she was doing her research into the healing power of prayer and, it appears, quite possibly sorting the data on prayer and recovery rates in ways that would lend credence to her hypothesis, even if she did so unconsciously.
This mystic who captured the attention of a public eager to believe and a media eager to accommodate them abandoned conventional treatment and died. Most reporters overlooked the irony. The Viking helmet that she wore toward the end to augment her healing powers never made it onto the evening news.
With many mainstream media outlets sympathetic to alternative medicine (Newsweek, for instance, ran a lengthy, positive special section on altmed recently), it is important to avoid being lulled into taking mind-over-matter claims for granted. There's nothing wrong with having a positive attitude and it may well help the body in some ways but, as the prayer says, we should have the wisdom to know the difference between the things we can change and the things we cannot.
