a1 Department of Clinical Neuroscience, Karolinska Institutet, Division of Psychiatry, Stockholm, Sweden
a2 Institute of Neuroscience and Physiology, Section of Psychiatry and Neurochemistry, Sahlgrenska Academy at Göteborg University, Göteborg, Sweden
a3 Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden
a4 Centre for Violence Prevention, Karolinska Institutet, Stockholm, Sweden
Background Research suggests that suicidal behaviour is aggregated in families. However, due to methodological limitations, including small sample sizes, the strength and pattern of this aggregation remains uncertain.
Method We examined the familial clustering of completed suicide in a Swedish total population sample. We linked the Cause of Death and Multi-Generation Registers and compared suicide rates among relatives of all 83 951 suicide decedents from 1952–2003 with those among relatives of population controls.
Results Patterns of familial aggregation of suicide among relatives to suicide decedents suggested genetic influences on suicide risk; the risk among full siblings (odds ratio 3.1, 95% confidence interval 2.8–3.5, 50% genetic similarity) was higher than that for maternal half-siblings (1.7, 1.1–2.7, 25% genetic similarity), despite similar environmental exposure. Further, monozygotic twins (100% genetic similarity) had a higher risk than dizygotic twins (50% genetic similarity) and cousins (12.5% genetic similarity) had higher suicide risk than controls. Shared (familial) environmental influences were also indicated; siblings to suicide decedents had a higher risk than offspring (both 50% genetically identical but siblings having a more shared environment, 3.1, 2.8–3.5 v. 2.0, 1.9–2.2), and maternal half-siblings had a higher risk than paternal half-siblings (both 50% genetically identical but the former with a more shared environment). Although comparisons of twins and half-siblings had overlapping confidence intervals, they were supported by sensitivity analyses, also including suicide attempts.
Conclusions Familial clustering of suicide is primarily influenced by genetic and also shared environmental factors. The family history of suicide should be considered when assessing suicide risk in clinical settings or designing and administering preventive interventions.
(Received December 03 2010)
(Revised April 28 2011)
(Accepted April 30 2011)
(Online publication June 01 2011)