A review of the literature indicates that scientists are only beginning to understand the role of serotonin levels in influencing mental states and behavior. Research findings to date suggest that serotonin levels could be linked to many problems such as low self-esteem, learning problems, and crime and violence. The effects of serotonin on aggression and violent behavior in police officers is currently unclear. It is apparent, however, that serotonin does have a major effect on impulsivity and aggression. This paper does not suggest that all violent behavior by police officers or any individual is caused entirely by low levels of serotonin; however, the literature surveyed does suggest the possibility that placing the blame on the long-held belief about pre-employment screening's inability to weed out the "rotten apples" may not be valid. Just as there is a need for longitudinal psychological testing of police officers at entry level as well as at various intervals of police service to determine continued suitability for the job, so should there by testing of serotonin levels in police applicants when they enter policing and at various intervals of their service. 15 references