Individuals who self-harm and engage in violence have overlapping aetiologies but are typically investigated as two discrete populations. We aimed to examine risk of unnatural death among persons with a history of self-harm and violent crime, focusing specifically on those with co-occurring behaviours.
Population-based nested case-control study using national interlinked Danish registers. 2246 people aged 35 or under who died unnaturally (cases) were matched by age and gender to 44,920 living persons (controls). We compared risks of suicide, accidental death and any death by external cause among those with a history of hospital-treated self-harm, violent criminality and both behaviours to individuals without histories of either behaviour. We estimated incidence rate ratios (IRRs), adjusted for age and gender, to compare risks.
1499/2246 cases (66·7%) died from accidental causes and 604 (26·9%) died by suicide. Risk of unnatural death was elevated for individuals with history of violence (IRR 5·2, 95% CI 4·4, 6·1) or self-harm (IRR 12·6, CI 10·8, 14·8), but the greatest risk elevation was among those with histories of both behaviours (IRR 29·4, CI 23·1, 37·4). Substance misuse disorder, particularly multiple drug use, was more prevalent among individuals with co-occurring self-harm and violence compared to individuals engaging in just one of these behaviours. Psychiatric disorder appeared to account for some of the excess risk of unnatural death among people with dual harm histories but excess risk, particularly of accidental death, persisted in the multivariable models.
Among persons with co-occurring self-harm and violence, risk of accidental death, particularly accidental self-poisoning, should be considered with equal importance as suicide risk. People who also have a substance misuse disorder are at particularly high risk of dying from external causes. Treatment strategies should be designed to be accessible for those facing turbulent lives with multiple problems. Individuals in this group will have been treated by healthcare services for self-harm and had contact with criminal justice services, providing multiple opportunities for proactive intervention.
European Research Council starting grant (ref. 335905) (Prof Webb).