The minds of killers who kill impulsively and murderers who carry out premeditated crimes differ both psychologically and intellectually, reveals a new study.
Robert Hanlon, senior author of the study and associate professor of clinical psychiatry and clinical neurology at Northwestern University Feinberg School of Medicine, said that impulsive murderers were much more mentally impaired, particularly cognitively impaired, in terms of both their intelligence and other cognitive functions.
He said that the predatory and premeditated murderers did not typically show any major intellectual or cognitive impairment, but many more of them have psychiatric disorders.
The study found that 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.
And 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.
Based on established criteria, 77 murderers from typical prison populations in Illinois and Missouri were classified into the two groups (affective/impulsive and premeditated/predatory murderers).
Hanlon compared their performances on standardized measures of intelligence and neuropsychological tests of memory, attention and executive functions.
He spent hours with each individual, administering series of tests to complete an evaluation. Hanlon has spent thousands of hours studying the minds of murderers through his research.
The study has been published online in the journal Criminal Justice and Behaviour.