Strengths of the study are the highly reliable, autopsy-based suicide determination, rigorous controls for potential confounding factors with complete, registry-based psychiatric and medical illness diagnosis and treatment information, the long follow-up period for suicide outcomes that increases confidence that the observed association is not due to pre-existing conditions that increase the risk of suicide, e.g. psychiatric co-morbidity hypothesis, or to the fact that tobacco smoking causes painful and debilitating diseases such as cancer and increases risk for suicide through that mechanism, e.g. the medical co-morbidity hypothesis, and the ability to conduct a within-twin-pair analysis of the association.
