Serotonin is definitely linked in research to depression. I believe the primary effect is dyscontrol, however, which means impulsiveness. Through that there is a higher risk for suicide or homicide. Antidepressant medications of the SSRI-type increases whatever serotonin is available and could help. Diet is also important. Serotonin is made from an amino acid called tryptophan which one can get from the right foods like turkey or in pill form. Tryptophan is a large amino acid, however, and doesn't compete well with smaller ones in crossing the blood-brain barrier into the brain unless one makes further adjustments in food intake. Something sweet like orange juice creates glucose (sugar) in the blood which causes insulin to be released. Insulin in turn causes the smaller amino acids to be taken into cells so that the circulating tryptophan have easier access to pass through the BBB. A husband and wife team (his name is Robert Wurtman) did a lot of work on this at MIT and have published books so you might check your library. Psychiatric sites could also give you info on SSRI. Lastly, search the web for info on Tryptophan. Good luck.