Partners in crime: A 21-year cohort comparison of people who commit serious crimes together with those who act alone

Perpetrators who act together violently occur frequently in police and media discussions, but are rarely the focus of forensic psychiatric research.

We aimed to characterise people who act together when committing a serious crime and to map the frequency of such crimes over 21 years in Finland.

Data for the study were retrieved from the national database of forensic psychiatric examinations for the period 2000-2020, with reports on file for nearly all people charged with serious criminal offences in the country. Index cases were defined as those with two or more perpetrators attacking a single victim; people who acted alone were comparison cases. Sex and age at the time of the crime were extracted together with all diagnoses listed in the reports.

Seventy-five multiple perpetrator groups (MPG) were identified, accounting for 165 individuals whose reports were compared with 2494 single-perpetrator (SPR) reports. Most group and solitary offenders were male (87%: 86%, respectively). The index offence was more likely to be homicide among the group perpetrators (mean 1.12) than the solitary offenders (mean 0.83). Proportionately more of the group offenders had personality disorder or substance use disorders (antisocial personality disorder MPG 49%: SPR 32%; any personality disorder MPG 89%: SPR 76%); alcohol (MPG 79%: SPR 69%; cannabis MPG 15%: SPR 9%). By contrast, psychosis was about twice as common among the solitary offenders (MPG 12%; SPR 26%).

The number of group-perpetrated crimes has not increased, according to these Finnish forensic psychiatric report data of 2000-2020, but the relatively high prevalence among them of personality and substance use disorders is a constant. Understanding psychiatric disorders as factors in both leading to and avoiding violent conflicts may help plan new approaches to further diminish group violence.