Background: Familial clustering of suicidal behaviour has been well documented. We aimed to assess whether family history of completed suicide and mental illness that results in admission to hospital are risk factors for suicide, and whether these factors interact.
Methods: We did a nested case-control study using data from Danish longitudinal registers. We included 4262 people who had committed suicide aged 9-45 years during 1981-97 (cases), and 80238 population-based controls matched for age, sex, and date of suicide. Suicide and psychiatric history of parents and siblings and socioeconomic, demographic, and psychiatric data for every case were retrieved and merged. Data were analysed with conditional logistic regression.
Findings: A family history of completed suicide and psychiatric illness significantly and independently increased suicide risk (odds ratio 2.58 [95% CI 1.84-3.61] and 1.31 [1.19-1.45], respectively). These effects were not accounted for by the socioeconomic status and psychiatric history of cases. A history of family psychiatric illness significantly raised suicide risk only in people without a history of psychiatric illness (1.55 [1.38-1.75]), whereas a family history of suicide increased suicide risk irrespective of psychiatric illness (2.37 [1.11-5.09] and 2.66 [1.82-3.88]) for people with and without a psychiatric history, respectively.
Interpretation: Completed suicide and psychiatric illness in relatives are risk factors for suicide, and the effect of family suicide history is independent of the familial cluster of mental disorders. Family history of suicide should be established in the assessment of suicide risk.