Objective Suicidal behaviours are poorly understood, making suicides hard to predict and prevent. This study aimed to use the largest twin study outside of the Western world to examine epidemiological correlates and genetic contributions to suicidal ideation, and its relationship with mental health problems. This is particularly pertinent in the context of a high suicide rate in Sri Lanka.

Method Active and passive suicidal ideation were assessed in a population-based twin registry (n=3957 twins) and a matched non-twin sample (n=2017). Logistic regression models were used to examine associations with sociodemographic factors, environmental exposures and mental health characteristics. The heritability of suicidal ideation was assessed using genetic model fitting in Mx statistical programme.

Results The prevalence of suicidal ideation was not different between twins and non-twins (contrary to previous reports), supporting the generalisability of results from twins to the general population. Factors that predicted suicidal ideation included female gender, termination of marital relationship, low level of education, urban residence, losing a parent while young, low standard of living, and stressful life events in the preceding 12 months. Suicidal ideation was strongly associated with depression, but also with abnormal fatigue, and alcohol and tobacco use. The best fitting genetic model indicated a strong contribution from genetic factors (57%; CI 47–66) and from environmental factors that are not shared across twins (43%; CI 34–53) in both men and women. In women this genetic component appeared to be largely mediated through depression, but in men there was a significant heritable component to suicidal ideation that was independent of depression.

Conclusion This study highlights environmental and sociodemographic characteristics that could be used to predict suicidal ideation in a high-prevalence country, although this study did not assess completed suicides. There is sparse prior research on the genetics of suicidal behaviours, and these are the first results to show a genetic contribution that is independent of depression. These results might represent peculiarities in Sri Lanka, but on the contrary our previous research highlights similarities between the aetiology of mental health in Sri Lanka and Western higher-income countries, supporting generalisability across cultural contexts.