Some researchers argue that exercise may act as a diversion from negative thoughts, and the mastery of a new skill may be important. There is evidence to indicate that social contact between people who are working out or involved in sports may be an important source of satisfaction as well. Still others think that physical activity causes the brain to release chemicals called endorphins that cause one to feel good after exercising. Most of the researchers looking at exercise and mood compared groups of people who were exercising to those who were not. They then looked to see if those who were exercising felt better in the short term. Some researchers compared exercising to treatments for depression such as antidepressant medications or cognitive-behavioral therapy. The vast majority of studies have shown that there is a significant association between exercise and improved well-being. It has proved more difficult, however, to show that exercise directly causes mental well-being; people who are happier, after all, may simply be more inclined to exercise.