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A new study on murderers reveals that they are not all the same.  The findings reveal that the minds of those who kill impulsively or out of rage are very different than the minds of those who carefully carry out premeditated crimes.

Lead researcher Robert Hanlon of Northwestern University wanted to look at the neuropsychological and intelligence differences of murderers who kill impulsively versus those who kill as the result of a premeditated strategic plan.

The study published in the journal Criminal Justice and Behavior, revealed a significant difference between these two types of murderers.

"Impulsive murderers were much more mentally impaired, particularly cognitively impaired, in terms of both their intelligence and other cognitive functions," Hanlon, an associate professor of clinical psychiatry and clinical neurology at Northwestern University Feinberg School of Medicine, said in a news release.

"The predatory and premeditated murderers did not typically show any major intellectual or cognitive impairments, but many more of them have psychiatric disorders," he said.

Researchers found that compared to impulsive murderers, premeditated murderers are almost twice as likely to have a history of mood disorders or psychotic disorders. The study revealed that 61 percent of the impulsive killers studied had a history of mood or psychotic disorders compared to 34 percent of premeditated killers.

The study found that impulsive murderers are also more likely to be developmentally disabled and have cognitive and intellectual impairments.  The findings revealed that 59 percent of impulsive murderers were disabled compared to 36 percent of premeditated murderers.

Researchers found that nearly all the impulsive murderers have a history of alcohol or drug abuse and/ or were intoxicated at the time of the crime.  The study revealed 93 percent of impulsive murderers had a drug problem compared to 76 percent of those who strategized about their crimes.

The study included 77 murderers from typical prison populations in Illinois and Missouri.  The murderers were classified into the two groups: affective/impulsive or premeditated/predatory murderers. Researcher compared their performances on standardized tests of intelligences and neuropsychological tests of memory, attention and executive functions.

"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," Hanlon said.

"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," he concluded.