Murder is a complex phenomenon that encompasses legal, psychological and investigative considerations. The main purpose of the various legal standards, specifically the degree of murder based on intentionality, is to assign punishment; the more intent involved, the greater the amount of punishment awarded. Psychiatric murderers are mentally disturbed, psychotic, or mentally retarded. Murders as a result of psychological disorders include various types of homicides such as sociogenic homicides, situational murders, homicides committed by impulsive offenders, and catathymic homicides. Compulsive murders stem from internal psychogenic sources; the need to commit the act is compelling, and there is a high likelihood of repetition. Another approach to understanding and classifying murder has been utilized by law enforcement and is based mainly on crime-scene characteristics. Categories of murder commonly used in the investigative approach are sexual murders, serial murders, mass murders, and spree killings.