Behaviorism began as a form of research psychology. For the early part of the 1900s, the behaviorists were strictly scientists. They worked in universities on psychological research problems. They did not really start to think about mental illness and therapy until the 1930s and 40s. This was well after the psychodynamic therapists had already taken over the field.
To the behaviorist, human behavior has nothing to do with internal unconscious conflicts, repression, or problems with object representations. A behavioral psychologist uses principles of how people learn to explain human behavior. Dysfunctional or unhelpful behavior such as depression is learned. Because depression is learned, behavioral psychologists suggest that it can also be unlearned.
In the mid-1970s, Peter Lewinsohn argued that depression is caused by a combination of stressors in a person's environment and a lack of personal skills. More specifically, the environmental stressors cause a person to receive less positive reinforcement. Positive reinforcement occurs when people do something they find pleasurable and rewarding. According to learning theory, receiving positive reinforcement increases the chances that people will repeat the sorts of actions they have taken that led them to receive that reinforcement. In other words, people will tend to repeat those behaviors that get reinforced. For example, many people show up at work on a regular basis in order to receive money or insurance benefits. Most children study in order to make sure that they will continue to receive good grades. In these examples, working and studying are behaviors that are motivated by money, benefits, and good grades, which are positive reinforcers.
According to Lewinsohn, people with depression are those who do not know how to cope with the fact that they are no longer receiving positive reinforcements like they were before. For example, a child who has moved to a new home and has lost touch with old friends might not have the social skills necessary to easily make new friends and could become depressed. Similarly, a man who has been fired from his job and has trouble finding a new job might become depressed.
In addition, people with depression typically have a heightened state of self-awareness about their lack of coping skills. This often leads them to criticize themselves and to withdraw from other people. They may avoid social events and get even less positive reinforcement than before. To make matters worse, some people with depression become positively reinforced for acting depressed when family members and social networks take pity on them and provide them with special support because they are "sick". For example, some spouses may take pity on their partners with depression. They may start to do their chores for them, while the person with depression lays in bed. If the person with depression was not thrilled to be doing those chores in the first place, remaining depressed so as to avoid having to do those chores might start to seem rewarding. Research suggests that Lewinsohn's theory explains the development of depression for some individuals, but not for all.
Traditionally, behaviorists did not pay much attention to people's thoughts, perceptions, evaluations or expectations. Instead, they focused solely on their external and directly observable and measurable behavior. They did this because they thought of thoughts as not very relevant to the process of influencing behavior, and also too difficult to measure with any accuracy. It turns out that this position was too extreme. More recently, research has shown that internal events such as perceptions, expectations, values, attitudes, personal evaluations of self and others, fears, desires, etc. do affect behavior. They are important to take into account when doing therapy. As a result, old-fashioned "strict" behavioral approaches to treating depression are not as popular today as in the past.