Murderers who kill intimate partners and family members have a significantly different psychological and forensic profile from murderers who kill people they don’t know, reports a new Northwestern Medicine study that examined the demographics, psychiatric history and neuropsychology of these individuals.

The new knowledge about murderers who commit what is called spontaneous domestic homicide — emotionally driven crimes that are not premeditated — could enable early intervention to prevent the homicide, the authors said.

Domestic homicide is one of the most common and frequent types of murder in the U.S.

One-third of all women murdered in the U.S. are killed by their male partners including husbands, ex-husbands, boyfriends and ex-boyfriends. An estimated 25 percent of women will be victims of severe domestic violence by an intimate partner in their lifetimes.

“The findings provide important information that may help prevent future domestic homicides, because they help identify individuals at risk of committing domestic murders,” said lead author Robert Hanlon, director of the forensic psychology research lab at Northwestern University Feinberg School of Medicine. “The killers in this group are very similar to each other and different from men who commit nondomestic murders, which are often premeditated.”

Hanlon also is an associate professor of psychiatry and behavioral sciences at Feinberg and a neuropsychologist at Northwestern Memorial Hospital.

The study on spontaneous domestic homicides found these killers have more severe mental illness (particularly psychotic disorders), few previous felony convictions, are less intelligent and have more cognitive impairment. The paper was published Aug. 21 in the early view online edition of the Journal of Forensic Sciences.

“These crimes are often preventable if family members are more informed about the potential danger from having someone who is severely mentally ill in the home and who may have shown violent tendencies in the past,” Hanlon said. “Family members may lull themselves into a state of false beliefs thinking ‘my son would never hurt me’ or ‘my husband may have a short fuse but he would never seriously harm me.'”

“The fact is the husband or son may very well harm the wife or mother,” Hanlon said.

These murders are not a premeditated, strategic type of killing, noted Hanlon, who testified in the James Holmes Colorado theater mass murder trial in Denver in July.

“These murders are in the heat of passion and generally involve drugs or alcohol and often are driven by jealousy or revenge following a separation or a split,” Hanlon said. “This is grabbing the kitchen knife out of the drawer in a fit of anger and stabbing her 42 times.”

Another scenario is a murder committed by a mentally ill son or another family member who is psychotic and thinks the victim is plotting against him.

Intimate partners and family members need to notify the authorities that they are concerned about potential harm and remove themselves from the situation, Hanlon recommended. “You can stay with relatives, call domestic violence hotlines and say, ‘I’m scared something is going to happen to me,'” Hanlon said. “Start the wheels turning and get assistance.”

For the study, Hanlon interviewed and personally evaluated 153 murderers for more than 1500 hours. Participants were men and women charged with and/or convicted of first-degree murder in Illinois, Missouri, Indiana, Colorado and Arizona.

“You learn a lot about them in that amount of time,” Hanlon said. “I saw the same patterns and trends over and over again.”