Murder by numbers: Researchers compare the crimes and minds of individual vs. multiple killers

Crime investigators such as the RCMP, FBI and even the CIA have powerful new knowledge at their disposal to potentially help solve murders, thanks to a ground-breaking study involving UBC Okanagan forensic psychology researchers. Their findings could help police generate predictions about the characteristics of a killer – or killers – based on the crime scene evidence and victim.

The study, Partners in Crime: A Comparison of Individual and Multi-Perpetrator Homicides, looked at 124 cases of convicted Canadian male offenders – a third of the cases involving multiple people during the crime – to determine what the crime scene could reveal about the nature of the suspect, and the likelihood of multiple perpetrators being involved.

“It was the first empirical study of this nature,” says Stephen Porter, professor of psychology at UBC Okanagan and a practicing forensic psychologist. “We really had no literature to draw upon to come up with predictions, so it was very exploratory in nature.”

Porter and the team analyzed victim characteristics, sexual violence, sadistic/gratuitous violence, instrumentality vs. reactivity (premeditated vs. spontaneous violence), motive, weapon use and the role of psychopathy.

“Although it was a carefully done scientific study, it has clear practical applications,” says Porter. “It can give police a better understanding of these types of crimes as well as offer suggestions regarding the type of suspect, or suspects, they are looking for in a particular case. It would help them get a quick sense of who they are dealing with.”

Quick indications about who to look for are important, Porter says, because in Canada 70 per cent of solved homicides between 1991 and 2005 were cleared within one week of the incident, with the likelihood of success dropping drastically after that time.

Porter’s co-investigators were Marcus Juodis, a PhD student at Dalhousie University with a background in domestic homicide reviews, Michael Woodworth, associate professor of psychology at UBC Okanagan, and Leanne ten Brinke, currently working on her PhD in the area of forensic psychology at UBC Okanagan. The team finished data analysis in 2008, and published the findings in the current issue of the psychology journal, Criminal Justice & Behavior.

“We were surprised to find that murders by single individuals were dramatically different from murders by multiple perpetrators in a number of ways,” says Porter.  “For example, there were demographic differences. When we looked at murder by multiple perpetrators, in general, these guys were younger – on average mid-20s – whereas the individual killers tended to be mid-30s, and ranging into their 40s, 50s and 60s.”

Some other findings suggested:

  • Multiple perpetrators tended to target male victims who were acquaintances – more than two-thirds of their victims were male adults.
  • The crimes of multiple perpetrators were generally premeditated, well-planned and with a clear goal in mind – often some sort of retribution, money or drugs.
  • Individual perpetrators were much more likely to target adult females, and although there was usually some level of premeditation, there was a lot of emotion and anger present that wasn’t found with the multiple perpetrator killers. They also were much more likely to engage in gratuitous violence, such as torture.
  • For individual male murderers, the victim tended to be either an adult stranger or a former partner.
  • The older males tended to act individually, while a significantly higher percentage of younger offenders acted with at least one accomplice.

“And one really interesting point that we found was that multiple perpetrators’ preferred method was to kill a victim with firearms and they were very unlikely to engage in ‘hands-on’ methods like strangulation,” says Porter. “We found the opposite pattern with individual killers, whose preferred method of homicide was strangulation or stabbing.”

When the researchers looked at how psychopathy played a role in a murder, they found that individual psychopaths typically used strangulation and left evidence of sadistic violence, even curiosity-driven post-mortem violence. They suspect that for multiple-perpetrator crimes, there will typically be a psychopathic individual who, through his keen manipulation skills, is able to convince peers to engage in heinous behavior.

“We already know something about how psychopaths do murders alone, but we suspect that with many multiple-perpetrator crimes there is probably a ringleader – a psychopathic fellow – who is convincing others to do his dirty work,” Porter says.

Both Porter and Woodworth strongly hope their research will reach law enforcement groups and other legal professionals. To learn more about this science, and abouty training opportunities, visit their websites and