Reinforcement contingency theory posits the belief that one cause of depression is the loss of reward contingencies that are adequate for the individual’s personal need. These rewards could be a satisfying job or the ability to engage in personally rewarding activities such as hobbies, sports, etc.
When these reward contingencies are lost, either through the loss of a job or the inability to engage in previously rewarding activities depression can be a result. As we know from classical conditioning, the loss of rewards for positive behavior leads to eventual extinction of behaviors. The extinction of a large repertoire of behaviors can lead to lack of responsiveness and and emotional/intellectual arousal and thus lead into depression.
In humans, this frequently occurs when a person reaches retirement age and finds themselves at home without the stimulation of work. When a person retires, and has no hobbies or other interests to fall back on, they can frequently suffer from this type of depression.