Optimism and positive thinking are generally seen as good while pessimism is considered bad. However, psychologist Gabriele Oettingen studied obese women who were starting a new diet and found the ones who spent more time daydreaming about being thinner actually lost 24 pounds less during the following year.
In her book Rethinking Positive Thinking: Inside the New Science of Motivation, Oettingen writes that the problem with positive fantasies appears to be their relaxing effect; daydreaming makes us feel good and this leads to less motivation for change.
Additionally, when more positive language appears in financial newspapers the DOW tends to decline more during the following week. Similarly, presidents who use more positive language about the future in their inaugural address tend to see lower GDP and higher unemployment during their tenure in office.

There are flaws in some of Oettingen’s studies so her findings should be considered carefully. However, I feel there is validity to the big idea: not all types of optimism are good. Thinking you will succeed at something (known as self-efficacy) appears helpful but thinking about yourself succeeding (positive fantasies) may be harmful.
Which type of optimism do you engage in more?