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It's in the Genes
Bipolar disorder appears to run in families across many generations. A person is 50% more likely to develop the mental health condition if a parent or sibling has the disorder. Studies of twins have revealed that an identical twin is twice as likely as a fraternal (non-identical) twin to have the disorder if the other twin is bipolar (Oltmans and Emery, 2007). The lifetime chance of an identical twin becoming bipolar if the other twin has the disorder is between 40% and 70%.
Half-siblings also have an increased risk of becoming bipolar, although this risk is significantly decreased compared to that of a full-blood sibling.
Studies have also been trying to zero in on a specific gene or genes that may increase an individual's risk of developing bipolar disorder. One genetic suspect is a gene called BDNF - brain-derived neurotropic factor. BDNF is a neuroprotein which is responsible for the development of many different neural processes such as neurogenesis. Neurogenesis is the process by which neurons (brain cells) are formed in the brain (Liu, 2009). In 2008, researchers Monteleone, Serritella, Martiadis, and Maj found that a high percentage of bipolar disorder people they studied had reduced levels of BDNF.
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Chemical imbalances in the brain are thought by some scientists to play a part in the onset of bipolar disorder. A chemical imbalance occurs when levels of one or several neurotransmitters are altered. The neurotransmitters norepinephrine, serotonin, and dopamine are thought to be the main brain chemicals involved in bipolar. For example, when norepinephrine levels are too high a person may start to experience symptoms of bipolar mania. If the levels are too low they may experience symptoms of bipolar depression (Kleinmann et al, 2003).
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A traumatic event, such as child abuse, has been shown by researchers to be closely related to the onset of both bipolar mania and bipolar depression. Research points to a connection between child abuse and bipolar disorder, with approximately 50% of bipolar individuals reporting incidents of abuse, both physical and mental, throughout childhood (Alloy, Abramson, Urosevic, Walshaw, Nusslock, and Neeren, 2005).
Grandin, Alloy and Abramson (2007) found in their research that individuals with bipolar disorder were more likely to have experienced stressful life events which contributed to the onset of the disorder. These include death of a close friend or relative, illness or drug addiction.
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Alloy, L.B., Abramson, L.Y., Urosevic, S., Walshaw, P.D., Nusslock, R., and Neeren, A.M., 2005. The psychosocial context of bipolar disorder: Environmental, cognitive, and developmental risk factors, Clinical Psychology Review 25, pp. 1043–1075
Grandin, L.D., Alloy, L.B., and Abramson, L.Y., 2007. Childhood stressful life events and bipolar spectrum disorders, Journal of Social and Clinical Psychology 26, pp. 460–478
Kleinman, L.S., Lowin, A., Flood, E.L., Gandhi, G., Edgell, E., and Revicki, D.E., 2003. Costs of bipolar disorder, PharmacoEconomics 21, pp. 601–622
Liu, R.T., 2010. 'Early life stressors and genetic influences on the development of bipolar disorder: The roles of childhood abuse and brain-derived neurotrophic factor', Child Abuse & Neglect, 34, 7, pp. 516-522
Monteleone, P., Serritella, C., Martiadis, V., and Maj, M, 2008. Decreased levels of serum brain-derived neurotrophic factor in both depressed and euthymic patients with unipolar depression and in euthymic patients with bipolar I and II disorders, Bipolar Disorders 10, pp. 95–100
What Causes Bipolar Disorder? http://www.cci.health.wa.gov.au/docs/Info-What%20causes%20bipolar%20disorder.pdf
Bipolar Disorder Guide http://www.webmd.com/bipolar-disorder/guide/bipolar-disorder-causes