A unique bond: Twin bereavement and lifespan associations of identical and fraternal twins

Analyses of twin mortality often use models with dependent unobserved frailty terms capturing genetic and childhood environmental determinants. This ignores that mortality rates can be co-dependent due to bereavement effects, that is, to a causal effect of the loss of the co-twin on the mortality rate of the surviving twin. We develop a novel approach based on a model incorporating both types of dependence. We prove identification and we estimate models with Danish register data on twin pairs. Among men, losing an identical co-twin at age 75 causally reduces the remaining lifetime on average by more than a year. This bereavement effect is less severe among non-identical twins. The results are relevant for the assessment of the economic component of bereavement effects in general. Furthermore, estimates of correlations between the frailty terms by zygosity and the ensuing relative importance of genetic determinants are highly dependent on whether bereavement effects are ignored.