Depression, like other mental illnesses, is probably caused by a combination of biological, environmental, and social factors, but the exact causes are not yet known. For years, scientists thought that low levels of certain neurotransmitters (such as serotonin, dopamine, or norepinephrine) in the brain caused depression. However, scientists now believe that the interplay of factors leading to depression is much more complex. Genetic causes have been suggested from family studies that have shown that between 20 and 50 percent of children and adolescents with depression have a family history of depression and that children of depressed parents are more than three times as likely as children with nondepressed parents to experience a depressive disorder. Abnormal endocrine function, specifically of the hypothalamus or pituitary, may play a role in causing depression. Other risk factors for depressive disorders in youths include stress, cigarette smoking, loss of a parent, the breakup of a romantic relationship, attention disorders, learning disorders, abuse, neglect, and other trauma including experiencing a natural disaster.