Symptoms of mania include irritability, an abnormally elevated mood with a decreased need for sleep, an exaggerated belief in one's own ability, excessive talking, and impulsive and often dangerous behavior. Genes and Environment Study after study suggests biochemical and genetic links to depression. A considerable amount of evidence supports the view that depressed people have imbalances in the brain's neurotransmitters, the chemicals that allow communication between nerve cells. Serotonin and norepinephrine are two neurotransmitters whose low levels are thought to play an especially important role. The fact that women have naturally lower serotonin levels than men may contribute to women's greater tendency to depression.
Family histories show a recurrence of depression from generation to generation.
Studies of identical twins confirm that depression and genes are related, finding that if one twin of an identical pair suffers from depression, the other has a 70 percent chance of developing the disease. For fraternal twins or siblings, the rate is just 25 percent. Environmental factors, however, may also play a role in depression. It was impossible for me to sleep, and I would wake up at 3 or 4 in the morning and literally shake. And when it was time to wake up, I just couldn't get out of bed.