The minds of murderers who kill impulsively, often out of
rage, and those who carefully carry out premeditated crimes differ markedly
both psychologically and intellectually, according to a new study by
Northwestern Medicine® researcher Robert Hanlon.
“Impulsive murderers were much more mentally impaired,
particularly cognitively impaired, in terms of both their intelligence and
other cognitive functions,” said Hanlon, senior author of the study and
associate professor of clinical psychiatry and clinical neurology at
Northwestern University Feinberg School of Medicine.
“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.
Differences between murderers
Published online in the journal Criminal Justice and Behaviour, the study is the first to examine
the neuropsychological and intelligence differences of murderers who kill
impulsively versus those who kill as the result of a premeditated strategic
Compared to impulsive murderers, premeditated murderers are
almost twice as likely to have a history of mood disorders or psychotic
disorders – 61% versus 34%.
Compared to predatory murderers, impulsive murderers are
more likely to be developmentally disabled and have cognitive and intellectual
impairments – 59% versus 36%.
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%
versus 76% of those who strategised 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 standardised 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.
“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,” he
said. “Ultimately, we may be able to increase our rates of prevention and also
assist the courts, particularly helping judges and juries to be more informed
about the minds and the mental abnormalities of the people who commit these