Psychologists have spent decades studying the brains of known serial killers in an attempt to discover what makes them tick. Are serial killers built just like the rest of us, or are their brains wired differently? What makes them become vicious, unrepentant killers?

Many serial killers suffer from antisocial personality disorder (APD), which does not usually have a clinical diagnosis. APD allows a killer to easily manipulate and charm people without the burden of empathy. However, some serial killers suffer from deeper mental illness, such as schizophrenia, bipolar disorder, or borderline personality disorder, which affects their ability to tell right from wrong.

When a serial killer is diagnosed, it makes it tough to work out whether they are fit to stand trial and face justice. Some people even believe that mentally ill serial killers should not be held responsible for their actions. For this reason, many killers go undiagnosed until after their trial (if at all). Then, the killers claim to be crazy in order to avoid the death penalty. In reality, although these murderers all are disturbed psychopaths, very few actually have a diagnosable mental illness.

This list includes famous murderers with antisocial personality disorders and paranoid schizophrenic killers. So which ones are actually insane? Find out below.