There is real power in understanding our bad feelings.  Negative emotions are useful in certain situations, yet can become overwhelming. Low moods prevent us from wasting effort in pursuit of unreachable goals, but they often escalate into pathological depression.  Understanding bad feelings by understanding individuals as individuals can reveal new paths for relieving human suffering.  Steve talks with Randolph M. Nesse, M.D., author of Good Reasons for Bad Feelings: Insights from the Frontier of Evolutionary Psychology.