A serial killer is a person who murders three or more persons in at least three separate events—with a “cooling off” period between kills. Serial killers generally kill during periods of stress—and after killing, they tend to feel temporary relief. While their motivation for killing varies, they typically fall into one of a few categories: obtaining money, experiencing the thrill and power, and ridding the world of evil-doers (in a very evil way, of course). A study of a group of 50 serial killers shows that the majority experienced abuse as children, including such abuse as neglect, sexual abuse, physical abuse, and psychological abuse.

Dr. Helen Morrison has studied and interviewed 135 serial killers—she found that they have shocking similarities, and that chromosome abnormality, which begins to express itself during puberty, is a likely trigger. Studies show that serial killers never develop a sense of attachment and belonging to the world. The lack of development means that serial killers are unable to develop emotional attachment to their victims, which allows them to “experiment” on them.

Jim Fallon, a neuroscientist who has studied the brains of psychopaths for over 20 years, discovered that “people with low orbital cortex activity are either free-wheeling types or sociopaths.” To learn more about the characteristics and mindset of serial killers, check out the infographic below.

Read more: Top 10 Time Killers