This mood disorder is characterized by alternating states of major depression and mania characterized by low energy, impaired motivation and need for sleep, feelings of extreme sadness, failure, worthlessness, and hopelessness; while mania is accompanied by elevations in energy and mood, with racing thoughts and speech, decreased need for sleep, grandiosity, and enhanced risk taking. About half of individuals with bipolar disorder experience psychotic symptoms, more often during acute mania than depression.  The disorder affects an estimated 1% of the population in its most severe form and up to 6% when it is in the spectrum of related temperamental conditions. Although typically a life-long disorder, there can be periods of remission. Genome-wide association studies demonstrate susceptibility genes accounting for genetic inheritance in up to 90% of cases, and variance in the genetic architecture that confers an additional risk of psychotic symptoms. However the mechanisms underlying the disorder are complex and remain largely unknown.

The notion that no great genius ever existed without a strain of madness has paved the way for an association linking creativity and bipolar disorder, with a ten-fold increase in the rate of bipolar disorder among artists as compared to the general population, and anecdotal association of the two in such individuals as Vincent van Gogh, F. Scott Fitzgerald, Ernest Hemingway, Walt Whitman, Rachmaninoff, Tchaikovsky and others who struggled with bipolar illness. One Swedish population-based study demonstrated an overrepresentation of individuals in creative occupations among bipolar patients and healthy first-degree relatives, while another investigation found a disproportionate concentration of individuals with bipolar disorder in creative occupations.  It may be said that relying on a creative occupation to serve as a proxy for creativity may introduce unfair bias; so too may the impulsivity associated with bipolar disorder lead to trouble in sustaining stable ordinary (non-creative) employment.

The space between madness and genius is becoming an area of increasing interest for behaviorists and clinicians. Several temperament and personality traits found in highly creative individuals with bipolar disorder are present in creative healthy individuals.  Both bipolar and creative individuals tend toward neuroticism, extraversion, and personality openness compared with noncreative controls.  Further, creative people who tend toward divergent thinking and over-inclusive cognitive style involving remote associations to facilitate originality, could be considered manic in nature. However the latter results instead from a failure to filter irrelevant stimuli from the environment, a process known as cognitive disinhibition. The combination of a high IQ and cognitive disinhibition which predicts creative achievement, can be found in many patients with bipolar disorder. 

Finally, contrary to evolutionary theory that genetic disorders associated with a loss of fitness are presumably pruned from the gene pool through the process of natural selection, bipolar disorder has persisted in the population with high heritability and stable prevalence. This is likely a result of the presence of many genes and regulatory regions for the disorder, including rare structural and de novo genetic variants accounting for schizophrenia susceptibility.  Notwithstanding, there may be traits of bipolar disorder that translate into fitness advantages in terms of social skills, affective temperament, and creative success that provide favorable evolutionary properties that serve to maintain bipolar risk alleles in the population.