Knife
Northwestern University found that men who kill in the heat of the moment are more likely to be psychoticGetty Images

What's the difference between a man who murders his family and a man who kills strangers? Although it may sound like the beginning of an inappropriate joke, researchers from Northwestern University have discovered that the psychological and forensic profiles of the two are hugely contrasting.

The new report published in the Journal of Forensic Sciences found that murderers, mostly men, who commit spontaneous domestic homicide typically have more severe mental illnesses and fewer felony convictions, are less intelligent and have a higher level of cognitive impairment.

It is thought that a quarter of US women will be the victim of severe domestic abuse in their lifetimes, while one-third of all murdered women in the US are killed at the hands of their boyfriend or husband, the report states.

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

"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. 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 adds that domestic murders are not usually premeditated and are often the result of drugs or a crime of passion. "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. This is grabbing the kitchen knife out of the drawer in a fit of anger and stabbing her 42 times."

For the research, Hanlon dedicated 1,500 hours to interviewing 153 convicted murderers of both sexes. "You learn a lot about them in that amount of time," he said. "I saw the same patterns and trends over and over again."