analysis controlling for unmeasured genetic and environmental factors, in 24 of the 28 twin pairs discordant for both smoking and suicide, the twin that died from suicide was the smoker. An experimental design in which some smokers were assigned to become abstinent would be required to understand whether smoking is a causal risk factor. In this study, smoking cessation was associated with reduced risk. However, we cannot rule out whether person-level characteristics associated with successfully quitting smoking are also associated with reduced risk for suicide. The findings are concordant with reports that public policies that reduce smoking on a population level are associated with reduced suicide rates in locations where they are implemented (Grucza et al. 2014), supporting a causal relationship, but it is possible that there are societal factors in these geographic areas that reduce suicide risk.
