Domestic homicide is one of the most common types of murder committed in the U.S. One-third of all women murdered in the country are killed by male partners — husbands, boyfriends, exes. It’s estimated that about 25 percent of women will be the victim of severe domestic violence by an intimate partner in their lifetime.

Murderers who kill their spouses, however, may be very psychologically different from those who kill people they don’t know. A new study coming out of Northwestern University examined the neuropsychology and psychiatric history of these individuals along with the demographics of their crimes, and provided some new knowledge about this type of homicide.

For the study, researchers personally interviewed and evaluated 153 murderers for over 1,500 hours. Participants were men and women who were either charged or convicted with first-degree murder in Illinois, Missouri, Indiana, Colorado, or Arizona.

Killers who commit spontaneous domestic homicide — meaning an emotionally driven crime that is not premeditated — were found to have more severe mental illness (particularly psychotic disorders) than those who commit other homicides. The study found that they were also less intelligent, had more cognitive impairment, and had few previous felony convictions.

"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, in a press release. "The killers in this group are very similar to each other and different from men who commit nondomestic murders, which are often premeditated."

Hanlon said that a husband or son may be very capable of harming a wife or mother, and that family members need to accept that.

"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.'"

Hanlon testified in the James Holmes Colorado theater shooting trial in July and said that domestic killings are not the same kind of strategic, premeditated murders.

"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."

Such a horrific act could potentially be prevented by family members informing the authorities if they are concerned about a person, and removing themselves from the situation. Hanlon recommends taking action by staying with relatives and calling a domestic hotline.

Source: Hanlon R, Brook M, Demery J, Cunningham M. Domestic Homicide: Neuropsychological Profiles of Murderers Who Kill Family Members and Intimate Partners. Journal of Forensic Sciences. 2015.