Between 1978 and 1980, Gerald and Charlene Gallego killed 10 victims. Most were teenagers, whom they kept as sex slaves before killing them. Gerald died of cancer in 2002, and Charlene -- who has since changed her name -- is now out of prison after serving her sentence.
Researchers believe that the minds of murderers who kill impulsively, often out of rage, and those like the Gallegos who carried out premeditated crimes, differ markedly both psychologically and intellectually.
"Impulsive murderers were much more mentally impaired, particularly cognitively impaired, in terms of both their intelligence and other cognitive functions," said Robert Hanlon of Northwestern University's Feinberg School of Medicine. He was senior author of a study, published in the journal Criminal Justice and Behavior, which investigated the minds of murderers.
"The predatory and premeditated murderers did not typically show any major intellectual or cognitive impairments, but many more of them have psychiatric disorders," Hanlon added.
Mass Shootings Have Long History