Among the doubly discordant pairs, the co-twin was alive at the end of follow-up in 22 pairs, while in five pairs, the co-twin had died from natural causes. Only one pair (MZ) was such that the co-twin, who was an occasional smoker, had death from accidental poisoning (not considered suicide), while the suicide case was the never smoker. We ran a pairwise survival model in which the hazard for the smoking twins was compared with the hazard of the non-smoking co-twin, and the HR was 5.5 (95% CI 1.90–16.0, p = 0.002), with HRs of 7.0 and 4.3 for MZ and DZ pairs, respectively.
