Psychiatric and somatic co-morbidity was assessed with national registry queries. Receipt of antipsychotic medication at any time from 1975 to 2004 and receipt of antidepressant medication from 1995 to 2004 was identified through the Social Insurance Institution (SII) of Finland (Cannon et al. 1998). Antidepressant use was used as an indicator of depressive illness because the psychiatric diagnosis for which an antidepressant was prescribed was not systematically recorded until 2000. Eligibility for psychiatric disability pension at any time from 1975 to 2004 was identified through Finnish pension registers (Harkonmaki et al. 2008). Twins with smoking-related somatic illnesses that increase suicide risk, such as cancer, diabetes, cardiovascular and pulmonary diseases such as emphysema, asthma and chronic bronchitis, were identified using self-report and Finnish medical registers to 2011. People with psychiatric or somatic illness were removed from the sample to create a healthy subcohort without tobacco-related somatic conditions for some sensitivity analyses (see Tables 3 and 4).
