Aggression can be self-directed suicidal behavior. There is evidence that reduced serotonin system function is related to suicidal behavior. Reduced serotonergic input may be associated with a lowered threshold for suicidal behavior. The threshold is of little functional consequence unless a psychiatric disorder with a suicidal ideation develops. Genetic factors involved in external & self-directed aggression may operate via lower serotonin activity. Neurobiology is only part of the solution.