The exact etiology of depression is yet to be determined; however, multiple factors, including biological, psychological, and environmental factors are involved in the presentation of depression. For example, an individual who has a first-degree relative with depression has a four times higher risk of developing depression than the general population. Twin studies have shown that an individual with a monozygotic twin with depression has as high as a fifty percent chance of developing the disorder. Major depression is caused by imbalance of certain neurotransmitters (chemical messengers) in the brain, such as serotonin, norepinephrine, and dopamine. Antidepressants work either by changing the sensitivity of the receptors or by increasing the availability of neurotransmitters in the brain. In addition to genetic components, there are many psychosocial factors that contribute to the development of mood disorders. For example, an individual with little or no social support will have fewer resources to handle stress and thereby will be at a greater risk of developing a mood disorder.