Modern researchers have not yet determined what causes depression, but the mood disorder appears to be related to elevated stress and life changes, and may be influenced by brain chemicals. Although the precise cause of depression is unknown, there are a number of theories that attempt to explain how it occurs. Proponents of psychoanalytic theory generally maintain that depression is the result of an unconscious problem, and that depressed people turn their anger and hostility inwards in order to avoid conflict with the outside world. Learning theorists suggest that depression is the result of learning to be helpless. That is, when people come to believe they have no control over their lives, socially, economically, or psychologically, they tend to give up trying and depression sets in. Psychologists who favor cognitive theories generally argue that when a person experiences self-defeating thoughts or dwells on negative thoughts for too long, low self-esteem and depression occur as a result. Biological theorists suggest that depression is influenced by the physiological mechanisms of the body, such as chemical activity in the brain. For more information about the causes of depression, consult a healthcare provider in your area.