Twins Exhibit Slightly Lower Mortality Rates than Non-Twins

Researchers here report on an interesting finding emerging from epidemiological data on twins, in that twins exhibit modestly lower mortality rates than the rest of the population. The paper focuses on the support and relationship angle, referencing the marriage effect on life expectancy, but I think that one could just as well field arguments based on effects in the womb, statistical differences in physical robustness, or a number of other items linked to longevity in human or animal studies that have shown up in the literature over the past few decades. Ultimately this is all interesting but irrelevant to the future of human longevity: small natural differences will be overwhelmed by the results of progress in medicine if things go well. A few years either way won't much matter when rejuvenation therapies can add decades of healthy life, something that may well happen within our lifetime if enough support goes to the right lines of research.

While studies of extreme longevity clustered within human families have indicated at least some genetic role in determining lifespan at very advanced ages, twin studies, which offer the opportunity to disentangle the genetic and environmental factors for a given trait, indicate genetic factors are responsible for only a modest amount of the variation (20-30%) in human lifespan and that the role of genetic factors is minimal before age 60, but increases thereafter. Although twin studies that focus on the correlation in age-at-death have yielded important insights into the role of genetics in human lifespan, the determinants of human survival patterns are immensely complex and change with age - i.e. while genetic factors play an increasingly larger role at advanced ages, environmental, social, and behavioral factors influence survival patterns much more heavily at younger ages. Perhaps owing to this complexity and the traditional structure of twin survival studies, less is known about differences in survival across age by zygosity, the underlying mortality processes that produce these differences, or the role of zygosity itself in shaping age patterns of survival.

Using data from the Danish Twin Registry and the Human Mortality Database, we show that monozygotic (MZ) twins have greater cumulative survival proportions at nearly every age compared to dizygotic (DZ) twins and the Danish general population. We examine this survival advantage by fitting these data with a two-process mortality model that partitions survivorship patterns into extrinsic and intrinsic mortality processes roughly corresponding to acute, environmental and chronic, biological origins. Overall, we find a survival advantage for MZ twins over DZ twins of both sexes at nearly every age and of DZ twins over the general population, but that different processes confer these advantages at different ages. For females, the survival advantage at all ages can be attributed to lower extrinsic mortality rates. Among males, extrinsic advantages account for the survival advantage up to about age 65 where the overall survival advantage begins to narrow and MZ males show better intrinsic survival than DZ males and DZ males show better intrinsic survival compared to the general population.

This research has documented a 'twin protection effect' akin to a marriage protection effect where a socially close relationship contributes to better survival outcomes throughout most of life. Notably, while we find evidence for a health protection effect arising from zygosity, the use of twin data allows us to avoid the confounding issue of self-selection that studies of marriage and health often encounter. Research on marriage protection effects as well as the findings presented in this paper are part of a larger body of literature that documents the importance of social support and cohesion for mortality and longevity outcomes. In this case greater survival for MZ twins over DZ twins and DZ twins over the general population is driven by lower extrinsic mortality at most ages, which is a likely consequence of the social bond between twins buffering against risky behaviors, providing emotional or material assistance during times of stress exposure, and promoting health-enhancing behaviors.

Link: http://dx.doi.org/10.1371/journal.pone.0154774

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