Frequently I have read anecdotal accounts of people who were sure that they were going to beat a life threatening disease (usually cancer) against the odds and managed to do so. Often they would give credit to their optimistic attitude that kept them fighting. There is not much thorough research that looks at long term effects of optimism and pessimism on health. However, there was one notable study that followed 99 men from age 25 to 60. Each man’s optimism was rated based on the way they explained bad events. If they thought that the cause of bad events were unstable, specific to the individual events and external to them rather than a result of their action, they were judged as optimists. Those who held the other view – the cause of bad events was stable, global and related to themselves – were labeled pessimists. Of course like most studies on this subject they drew from a pool of subjects that would tend to have a larger number of optimists who assumed they had better than average prospects, namely students at a prestigious private college. In this case it was Harvard 
These researchers (Peterson, Seligman and Vaillant) found that a pessimistic attitude predicted poorer health from ages 45 through 60. They were unable to pinpoint the specific cause for this. Some suggested reasons were lower immune system or less support from other people. Evidently pessimists are not as popular among their peers, or perhaps they just don’t want to rely on other people as much. The pessimists definitely made more frequent visits to the doctor. However, as these men reached their fifties the correlation between poorer health and pessimism began to fade. The study did not follow them beyond their sixties, but one wonders if this trend would continue.
Another study on health, longevity and outlook on life was conducted by Frieder R. Lang, of the University of Erlangen-Nuremberg in Germany. According to his research people who have low expectations for the future are more likely to live longer. His study was only ten years long and consisted of a much larger and diverse population, 40,000 people from ages 18 to 96. Each group were asked questions to predict how satisfying life would be in five years. Then, five years later they were asked the question again and again at the end of ten years (if they survived that long). For the oldest group, 65 and above, over 40% predicted life would be less satisfying than they actually found it five years later. This group had fewer disabilities than those that thought their life would be better than it turned out to be. Of course, just like the study on the health of Harvard men, the researchers had had only guesses as to why this occurred.
“Pessimism about the future may encourage people to live more carefully, taking health and safety precautions.”
However, I have my own ideas. This second study did not measure optimism or pessimism as much as it measured how well people could predict their future satisfaction with life. It may not just be an unfulfilled pessimistic forecast that results in healthier, longer lives. The fact that these aging people found themselves healthier than their peers may have led them to become more satisfied. In the same way those who suffered more disabilities would abandon an earlier optimistic attitude and be less happy with their lives. Most likely the decline in health had more of an effect on attitude than attitude did on health.
 Peterson, Christopher; Seligman, Martin E. P. Vaillant, George E. Pessimistic Explanatory Style Is a Risk Factor for Physical Illness: A Thirty-Five-Year Longitudinal Study, Journal of Personality and Social Psychology, 1988, Vol. 55, No. 1,23-27
 Lang, Frieder R. Weiss, David. Gerstorf, Denis. Wagner, Gert G. Forecasting Life Satisfaction Across Adulthood: Benefits of Seeing a Dark Future? Psychology and Aging © 2013 American Psychological Association 2013, Vol. 28, No. 1, 249 –26