In the summer of 1971, a team of researchers led by Psychology professor Phillips Zimbardo divided a group of undergraduates randomly into two groups, prisoners and prison guards, and arranged for them to act out their respective roles in a mock prison in the basement of the Stanford psychology building.

Within days, the guards began to display authoritarian attitudes, ultimately subjecting some of the prisoners to intentional . The prisoners developed passive attitudes, many sinking into a depressed state. The experiment had to be stopped after only six days.

The Stanford prison experiment is often said to illustrate the power of social roles in shaping behavior, but it also illustrates the power of behavior to elicit real powerful emotions. The guards in Zimbardo's experiments were not really guards. And the prisoners were not prisoners. They were all volunteers. They were all students. But once they began to act the part, they began to feel the part.

Many people assume that the link between emotion and behavior is one-way: Emotions shape behavior. You love him, therefore you kiss him. You hate him, therefore you hit him. This view is incorrect. In fact, the relationship is reciprocal. Much of the time, behavior actually shapes emotion.

Ever wonder why so often the actor and actress who play a couple in a movie fall in love on the set? Multiple processes are involved, to be sure. Both are usually young and attractive. They have much in common. They hang around each other a lot. All these are known predictors of mate selection.

But they also do love scenes together. They have to act like people who care deeply for each other. They look into each other's eyes, they touch each other. They act out the behaviors of love. No wonder the emotion of love often follows.

The psychologist/philosopher William James was one of the first theorists to notice this counter-intuitive process. He believed that emotions arise out of the bodily actions we take in response to what is happening in our lives. It is not, he theorized, that, "we lose our fortune, are sorry and weep; we meet a bear, are frightened and run; we are insulted by a rival and angry and strike." In fact, he argued, "this order of sequence is incorrect...the more rational statement is that we feel sorry because we cry, angry because we strike, afraid because we tremble."

James argued that without some kind of bodily response (crying, trembling, striking) we would not feel emotion. "We might then see the bear, and judge it best to run, receive the insult and deem it right to strike, but we should not actually feel afraid or angry." While over simplifying somewhat, he was still onto an essential truth. Behavior can create emotion.

Recent research in clinical psychology has shown that the fastest way to change an emotion is to change the behavior attached to it. The idea itself is not new. For example, behavioral theorists back in the 70s believed that was, indirectly, a result of inactivity: after many failures and disappointments, people stopped trying and withdrew from the world; withdrawal and inactivity, however, decrease the possibility of positive interactions or experiences, hence isolation and passivity increase, hence depression.

Human beings, prone as they are to prefer immediate rewards, often respond to discomfort by withdrawal and avoidance. Withdrawal and avoidance reward us in the short run by eliminating discomfort, but they punish us in the long run by preventing us from learning how to obtain rewards in . The correct reaction to failure is not to give up and shut yourself away, but to learn to act more skillfully and purposefully so as to reintroduce positive reinforcements into your life. Behavioral treatment for depression, then, revolves around getting the client to change behaviors in order to experience a change in mood-a notion referred to as behavioral activation.

Behavioral approaches to treating depression were pushed aside somewhat in the 80s by cognitive techniques, which focused on altering internal "cognitive distortions" (catastrophic, pessimistic thoughts) and negative "attributional styles" (self-punitive habits of assigning meaning to events). However, several studies in the 90s, showing that a behavioral activation component alone performed as well as the total cognitive package for depression, created renewed interest in the earlier ideas.

The behavioral activation model assumes that depression has to do with the sufferer's external circumstances, not merely with the sufferer's internal characteristics. Disorders, in other words, are ‘events in context.' The behavioral activation model hence represents a ‘contextualist' view of mental illness. Rather than reflecting the individual's internal genetic or cognitive flaws, disorders are seen as an interaction between individual characteristics and environmental conditions.

The principle that behavior shapes emotion is applied these days in the treatment of depression through a technique called ‘activity planning,' in which clients are asked to reintroduce into their lives activities that are associated with feelings of achievement and pleasure. This goes beyond past recommendations to ‘take a walk' or ‘go out on the town.' Therapists work with clients to look at the specific behavioral contingencies that exist in their lives, break tasks into small, easily attained steps, and build chains of reinforcement to elicit successful behavior in the world. The psychologist acts in a way that is analogous to the way a physical therapist breaks down movement into small component parts that can be easily practiced in order to build strength and flexibility.

Being active in the world may lead to changes in mood through several paths. Physical activity leads to a feeling of well-being by releasing pain reducing , not to mention increasing heart capacity and muscle strength, improving appearance, etc. Our bodies are built for motion, and they feel good moving.

Being active in the world also gets you to be around people. We are social animals, and social interaction has positive effects on our mood. The single best predictor of human is the quality of social relationships. Moreover, activity often amounts to practice, which improves skill, which improves our ability to obtain rewards in the environment.

The bottom line message is useful not just for people with depression, but for anyone interested in maintaining sound mental hygiene. The shortest, most reliable way to change how you're feeling is to change what you're doing. When you feel bad, don't wait to feel good to do what you love. Start doing what you love. Good feelings will likely follow.