The cheerful do not live longer. So conclude Friedman and Martin in The Longevity Project.
The cheerful are more likely to have better health in the short run. They are more likely to follow through with medical treatments and therapy regimens because they believe that things will work out well. Pessimists are more likely to quit early, or not even try, because they do not expect a good result.
Friedman and Martin speculate that optimists may be disheartened if things do not work out quickly, and so suffer worse stress or depression which negates the health advantage of cheerfulness.
I am in favor of cheerfulness. I believe, and teach to my kids and my students, that everything will work out just fine. I think Friedman and Martin make a mistake in conflating cheerfulness with optimism (and at one point with a "happy-go-lucky" attitude).
Optimists expect things to work out well. Cheerful people deal with reality with good cheer. Both attitudes help you get through illness better. But optimism can't fend off death. However, cheerfulness helps you deal with death better. SO optimism (which I think is what Friedman and Martin are really measuring) can have a good health effect without having a longevity effect.