Causation
The term
depression is often modified by words that imply either some specific factor or some
chemical mechanism as the cause of the state. Thus, in
psychological terms,
depressions have been considered as reactions to some loss of or separation from a valued person or object. These are called reactive depressions (or neurotic depressions, but that is a misnomer), as contrasted with the usually more severe depressions without apparent cause called endogenous depressions, or those accompanied by delusions or hallucinations and sometimes called psychotic depressions. (Melancholia, a term once used to describe all depressive states, is now applied only to these most severe forms.) Such distinctions are not clear-cut, however, and may not be useful when deciding on the mode of treatment of the depressed patient.
The predominant chemical, or
biological, theory of depression is the so-called amine hypothesis. Based on observations that depression can be caused by drugs that deplete the brain of chemicals called biogenic amines, the theory suggests that depressions are caused by a decrease in amines such as norepinephrine (noradrenaline) and serotonin in specific brain areas. In fact, most (but not all) effective antidepressant drugs can increase the amounts of amines in the brain by blocking their breakdown or exit from the body. Despite the theory's attractiveness, however, the exact nature of the presumed chemical lesion in the brain has yet to be defined even after many years of research.
Psychological theories of the cause of major depression remain equally unproved, however, and clear familial associations of depressive illnessÑalong with the responsiveness of the state to drugsÑdo indicate that depression has some biological substrate. Most modern theorists would agree that both biological and psychological factors play a role.