MANY people think that the key to success is to cultivate and doggedly maintain an optimistic outlook. This belief in the power of positive thinking, expressed with varying degrees of sophistication, informs everything from affirmative pop anthems like Katy Perry’s “Roar” to the Mayo Clinic’s suggestion that you may be able to improve your health by eliminating “negative self-talk.” But the truth is that positive thinking often hinders us. More than two decades ago, I conduct