I read this somewhere before. It does make sense in a way...

An optimist are risk-takers, they are more likely to take on crazy stunts, adventures, do more things which put their life at risks. Whereas a pessimist are fearful and more likely take precaution on their health/safety before doing something. They spend as much time as they can being in a safe environment, like at home etc.

It's a pretty stupid study in my opinion.