People with depression have a strong suicidal tendency, and many people think that depression is a mental illness. In the middle of the 20th century, medical research found that the concentration of three neurotransmitters (serotonin, norepinephrine and dopamine) in the brain of depressed suicides is lower than that of normal people. Medical scientists speculate that it is the imbalance in the concentration of these three neurotransmitters that leads to depression.
The statement that drugs developed to maintain the concentration balance of the three neurotransmitters are effective in the treatment of depression gives the strongest support to the conclusions of the medical scientists.