It is argued that humans have retained the instinct to experience low mood during the winter months, even if the availability of food is no longer determined by the weather.Much of what is known about the genetic influence of clinical depression is based upon research that has been done with identical twins. Identical twins have exactly the same genetic code. It has been found that when one identical twin becomes depressed the other will also develop clinical depression approximately 76% of the time. When identical twins are raised apart from each other, they will both become depressed about 67% of the time. Because both twins become depressed at such a high rate, the implication is that there is a strong genetic influence. If it happened that when one twin becomes clinically depressed the other always develops depression, then clinical depression would likely be entirely genetic.Bipolar disorder is also considered a mood disorder and it is hypothesized that it might be caused by mitochondrial dysfunction. Sex differences
Mood disorders, specifically stress-related mood disorders such as anxiety and depression, have been shown to have differing rates of diagnosis based on sex. In the United States, women are two times more likely than men to be diagnosed with a stress-related mood disorder. Underlying these sex differences, studies have shown a dysregulation of stress-responsive neuroendocrine function causing an increase in the likelihood of developing these affective disorders. Overactivation of the hypothalamic-pituitary-adrenal (HPA) axis could provide potential insight into how these sex differences arise.
Those in a manic state are more emotionally sensitive and show less inhibition about attitudes, which could create greater expression. Studies performed at Harvard looked into the amount of original thinking in solving creative tasks. Bipolar individuals, whose disorder was not severe, tended to show greater degrees of creativity".The relationship between depression and creativity appears to be especially strong among poets. See also
Personality disorder

References
Cited textsAmerican Psychiatric Association (2000). Diagnostic and statistical manual of mental disorders, Fourth Edition, Text Revision: DSM-IV-TR. Washington, DC: American Psychiatric Publishing, Inc. ISBN 978-0-89042-025-6. American Psychiatric Association (2013). Diagnostic and statistical manual of mental disorders, Fifth Edition, Text Revision: DSM-5. Washington, DC: American Psychiatric Association, Inc. ISBN 978-0-89042-554-1. Carlson, Neil R.; Heth, C. Donald (2007). Psychology the science of behaviour (4th ed.). Pearson Education Inc. ISBN 978-0-205-64524-4. Collier, Judith; Longmore, Murray (2003). "4". In Scally, Peter (ed.). Oxford Handbook of Clinical Specialties (6th ed.). Oxford University Press. ISBN 978-0-19-852518-9. Nesse, Randolphe M.; Williams, George C. (1994). Why We Get Sick: The New Science of Darwinian Medicine. Vintage Books. ISBN 0-8129-2224-7. Nolen-Hoeksema, S (2013). Abnormal Psychology (6th ed.). McGraw-Hill Higher Education. ISBN 9780077499693. Retrieved 5 December 2014. Parker, Gordon; Hadzi-Pavlovic, Dusan; Eyers, Kerrie (1996). Melancholia: A disorder of movement and mood: a phenomenological and neurobiological review. Cambridge: Cambridge University Press. ISBN 978-0-521-47275-3. Sadock, Benjamin J.; Sadock, Virginia A. (2002). Kaplan and Sadocks Synopsis of Psychiatry: Behavioral Sciences/Clinical Psychiatry (9th ed.). Lippincott Williams & Wilkins. ISBN 978-0-7817-3183-6. Semple, David; Smyth, Roger; Burns, Jonathan; Darjee, Rajan; McIntosh, Andrew (2007) [2005]. "13". Oxford Handbook of Psychiatry.