The minds of murderers who kill impulsively - crimes of passion, as they are commonly called - and those who carefully carry out premeditated crimes differ markedly both psychologically and intellectually, according to a new paper.

Predatory murderers had more psychiatric disorders, though not intellectual impairment. Adam Lanza and James Holmes fit that profile, as do many of the large planned homicides that have occurred in the last decade, whereas it may be that New England Patriots tight end Aaron Hernandez was an impulse killer - unless the investigation into another killing pans out.

The paper in Criminal Justice and Behavior examines the neuropsychological and intelligence differences of murderers who kill impulsively versus those who kill as the result of a premeditated strategic plan and found:

  • Compared to impulsive murderers, premeditated murderers are almost twice as likely to have a history of mood disorders or psychotic disorders -- 61 percent versus 34 percent.

  • Compared to predatory murderers, impulsive murderers are more likely to be developmentally disabled and have cognitive and intellectual impairments -- 59 percent versus 36 percent.

  • Nearly all of the impulsive murderers have a history of alcohol or drug abuse and/or were intoxicated at the time of the crime -- 93 percent versus 76 percent of those who strategized about their crimes.

"Impulsive murderers were much more mentally impaired, particularly cognitively impaired, in terms of both their intelligence and other cognitive functions," said Northwestern
researcher Robert Hanlon, senior author of the study and associate professor of clinical psychiatry and clinical neurology. "The predatory and premeditated murderers did not typically show any major intellectual or cognitive impairments, but many more of them have psychiatric disorders."

Based on current psychiatric criteria, 77 murderers from typical prison populations in Illinois and Missouri were classified into the two groups (affective/impulsive and premeditated/predatory murderers). Their performances on standardized measures of intelligence and neuropsychological tests of memory, attention and executive functions were compared and then Hanlon spent hours with each individual, administering series of tests to complete an evaluation. He estimates he spent thousands of hours studying the minds of murderers through his research.

"It's important to try to learn as much as we can about the thought patterns and the psychopathology, neuropathology and mental disorders that tend to characterize the types of people committing these crimes," said Hanlon. "Ultimately, we may be able to increase our rates of prevention and also assist the courts, particularly helping judges and juries be more informed about the minds and the mental abnormalities of the people who commit these violent crimes."