There is not just one cause of depression. It is a complex disease that can occur as a result of a multitude of different factors, including biology and emotional and environmental factors. For people biologically vulnerable to depression, it may sometimes start with a significant life event, such as the loss of a loved one or a change in one's life or after being diagnosed with a serious disease. For others, depression may just occur for no apparent "reason." In fact, there does not need to be any apparent "reason" for the symptoms of depression to occur in people who are vulnerable to the illness.
Depression is a type of mood disorder that some believe is triggered when neurotransmitters in the brain are out of balance. Neurotransmitters are chemical messengers that help the brain communicate with other parts of the body. These chemicals help regulate many physiological functions.
Low levels of neurotransmitters may play a role in why some people are more susceptible to depression, including the neurotransmitters:
Having an immediate family member with depression or a mood disorder can increase your risk for depression. The American Psychiatric Association (APA) states that if one identical twin is diagnosed with depression, the other twin has a 70 percent chance of developing it.
However, depression can occur in people with no family history, which is why some scientists believe it can be a product of both genes and life experiences.