A serial killer is a person who kills three or more people with a cooling off period between the killings, and these killings can go on for a period of months or years. The murders committed by this person may be similar or the victims may have something in common, such as occupation, race, appearance, gender or age range. Psychology is the science and study of human behavior and mental processes. Furthermore, it is the study of the human mind and its function. Krafft-Ebing (1886) found that the serial killer had suffered animal cruelty; enjoying the torture and pain of the victim during his childhood. Furthermore, the mothers of these serial killers spent most of their time working or doing other things and the father was usually absent. These children experience rejection and lack of attention, so they grow up with low self-esteem. Research shows that adults who experienced abuse and violent behavior during their childhood were three times more likely to become violent as adults than non-abused adults (Dutton & Hart, 1992). Freud (1940) was the first to establish the link between sexual abuse in childhood and abnormal behavior in adulthood. As a result, the serial killer uses sex as a way to vent his anger and aggression. The serial killer's sexual acts are not just about sex, they are about revenge, power, and control. “Serial killers unconsciously try to eliminate their repressed sexual pain and helplessness. Every stab into the victim's flesh is a stab against the terror and sexual pain of her childhood, and the rage that accompanies it is a rage against those who tormented and terrorized her” (Knight, 2006, p. 1199-1200 ). Aggression and the need to compensate for horrific childhood memories explain why serial killers abuse animals during their childhood. When they were children they controlled pets and could harm them knowing that animals can react. Therefore, they feel as if they hold absolute power. This animal torture will later turn into torture of human victims. Charister Clause and Lars Lidberg (1999) used five characteristics of Shahriar's disorder that are common among serial killers.