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Every day there are new studies linking our mental health to our physical health. Our moods or mental states - positive, neutral, negative - seem to be related to the risk of disease, and indeed, our likelihood of death.

Just last month, for example, a study reported that cardiovascular health is significantly better in people who report being happier. On one level, there is an obvious explanation to the phenomenon: Happy people are more likely to engage in the healthy behaviors - exercise and eating right - that lead to good hearts in the first place. While this relationship may have a lot of explanatory power, the...