It is primarily due to genetic vulnerability. It tends to run in families. Significant stressors can precipitate symptoms, whereas absence of these can lead to remission. In some individual with "incomplete penetrance" of relevant genes or "variable expressivity", bipolar disorder may be manifested only when stressors are particularly high.

Chromaticism

In music, the use of all 12 tones, especially for heightened expressivity. A standard key or mode principally employs 7 tones, leaving 5 tones for discretionary use. Use of all 12 tones in a given piece increased in the 18th and 19th centuries. Strictly controlled chromaticism, as in the ornamentation of Frédéric Chopin, did not threaten the perception of tonality. However, from the mid-19th century on, complaints were heard with ever greater frequency that it was difficult to perceive what a given piece's tonal centre was, the chromaticism in the works of Richard Wagner being the most notorious. The virtual breakdown in tonality in the works of advanced composers led to the free atonality of Arnold Schoenberg and his followers in the early 20th century.

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