Mass shootings account for a small fraction of annual worldwide murders, yet disproportionately affect society and influence policy. Evidence suggesting a link between mass shootings and severe mental illness (i.e. involving psychosis) is often misrepresented, generating stigma. Thus, the actual prevalence constitutes a key public health concern.

We examined global personal-cause mass murders from 1900 to 2019, amassed by review of 14 785 murders publicly described in English in print or online, and collected information regarding perpetrator, demographics, legal history, drug use and alcohol misuse, and history of symptoms of psychiatric or neurologic illness using standardized methods. We distinguished whether firearms were or were not used, and, if so, the type (non-automatic v. semi- or fully-automatic).

We identified 1315 mass murders, 65% of which involved firearms. Lifetime psychotic symptoms were noted among 11% of perpetrators, consistent with previous reports, including 18% of mass murderers who did not use firearms and 8% of those who did (χ2 = 28.0, p < 0.01). US-based mass shooters were more likely to have legal histories, use recreational drugs or misuse alcohol, or have histories of non-psychotic psychiatric or neurologic symptoms. US-based mass shooters with symptoms of any psychiatric or neurologic illness more frequently used semi-or fully-automatic firearms.

These results suggest that policies aimed at preventing mass shootings by focusing on serious mental illness, characterized by psychotic symptoms, may have limited impact. Policies such as those targeting firearm access, recreational drug use and alcohol misuse, legal history, and non-psychotic psychopathology might yield more substantial results.

Radicalization is a process, by which individuals adopt extreme political, social and religious ideation that leads to mass violence acts. It has been hypothesized that mental health characteristics might be associated with a risk of radicalization. However, a qualitative synthesis of studies investigating the relationship between mental health and radicalization has not been performed so far. Therefore, we aimed to perform a systematic review of studies examining the association between mental health characteristics and the risk of radicalization. Two reviewers performed an independent search of online databases from their inception until 8th April 2018 and 12 publications met eligibility criteria. There were several methodological limitations across the majority of eligible publications, including doubtful sample representativeness, use of diagnostic procedures without personal assessment of mental health status or lack of standardized tools for assessment of mental health. Representative cross-sectional studies revealed that depressive symptoms might be associated with radicalization proneness. However, it remains unknown whether depressive symptoms are associated with resilience or vulnerability to radicalization. Another finding from our systematic review is that several personality traits might predispose to develop extreme ideation. Finally, there is some evidence that lone-actors might represent a specific subgroup of subjects with extreme beliefs which can be characterized by high prevalence of psychotic and/or mood disorders. In conclusion, this systematic review indicates that caution should be taken on how the association between ‘mental health’ and ‘radicalization’ is being claimed, because of limited evidence so far, and a number of methodological limitations of studies addressing this issue.