Since major depression is a condition that is not yet fully understood, there is a lot of thinking about it that is shaped by general ideas rather than facts. In the medical literature, there are several ideas about depression that seem to be shaped by analogies to other conditions. The ways in which better understood conditions work, seems often to shape the ways in which people strive to understand more mysterious conditions such as major depression. In a recent paper, I have sought to describe eight such models. Since they are ideas that are usually based more on analogy than solid evidence, it is perhaps fair to call them "metaphors" rather than models or theories. Here is my list of eight such metaphors:
(1) as chemical imbalances (e.g., a presumed or theoretical imbalance of normally balanced neurotransmission in the brain).
(2) as degenerative conditions (e.g., a brain disease characterised by atrophy of specified brain structures).
(3) as toxicological syndromes (a result of exposure to a noxious psychological environment, e.g. "toxic stress").
(4) Injuries (e.g., externally induced brain damage related to damaging stressors).
(5) deficiency states (e.g., a serotonin deficiency).
(6) an obsolete category (e.g., similar to obsolete terms such as 'consumption' or 'dropsy').
(7) a medical mysteries (e.g., a condition poised for a paradigm-shifting breakthrough).
(8) or evolutionary vestiges (residual components of once adaptive mechanisms have become maladaptive in modern environments).
Each of these possibilities may have elements of truth within them and may ultimately blossom into full-fledged theories capable of effectively structuring knowledge. However, for now it is important to remember that the biomedical "pedigrees" of such ideas may provide them with more weight than they really deserve.