The Power of Positive Thinking

The Atlantic has an overview of the study of optimism.

...in 1985, Michael F. Scheier and Charles S. Carver's published their seminal study, "Optimism, Coping, and Health: Assessment and Implications of Generalized Outcome Expectancies" in Health Psychology. Researchers immediately embraced the simple hopefulness test they included in the paper and their work has now been cited in at least 3,145 other published works. Just as importantly, by testing the effect of a personality variable on a person's physical health, Scheier and Carver helped bridge the gap between the worlds of psychology and biology. After the paper, scientists had a method for seriously studying the healing powers of positive thinking.