The smaller than expected mortality was primarily due to the lesser mortality in the older birth cohorts. Among twins born in the 1940s and 1950s, the SMRs were very close to one (SMR= 99.9 for men and women combined). Because both twins in a pair had to be alive to be included in the cohort when it was compiled, the oldest ones in particular were probably healthier than average. This ‘healthy pair’ effect is equivalent to the health worker effect in occupational health studies. The SMRs by age, averaged over the 33 years of follow-up, are shown in the Figure separately for men and women. In most age-groups the mortality of twins is very similar to that of the population, with some mortality deficit in older age-groups. Thus, we find no evidence for excess mortality in twins compared to the population at large.
