Outline of a theory of manic-depressive illness.

Manic-depressive illness is characterized by locking into extreme states and alternation between those extremes. Locking into extreme states is also characteristic of a bistable, a basic electronic device of which the essential feature is a positive feedback loop with greater than unity gain. An argument is presented that under natural selection there has evolved a well-known and advantageous tendency for perceptions of successes to increase confidence, and that it is in the nature of confidence to increase perceptions of successes. Thus arises a positive feedback loop, in which perceptions of successes increase confidence, which further increases perceptions of successes, and so on, and the converse with failure and inconfidence. When the gain of the loop exceeds unity then locking into mania or depression will result. The feedback gain depends on both internal 'biological' factors and external situational factors. As a result of natural fluctuations the gain will occasionally fall temporarily below unity, releasing from the extreme state. Escape from mania will bias towards depression, whereas escape from depression will be elating. The physical embodiment of confidence may be serotonergic (or closely related) activity.