Why is Life Span Inherited to any Significant Degree?
Why do the life spans of parents exhibit some degree of correlation with the life spans of their children? "Genetics" is probably not an acceptable answer, given present evidence for natural genetic variation to contribute comparatively little to human life expectancy in all but a few rare cases. So is it cultural, where culture influences lifestyle choices closely correlated with health, such as weight gain or smoking? Or is it due to wealth effects, for much the same reasons? If so, then why is there such variation in life expectancy within specific social groups and wealth strata? These are tough questions to answer with any reliability given snapshot data from groups within human populations. Any given large study is just a single data point in the ongoing process of analysis and debate that spans decades and the entire scientific community.
In the long run, I have my doubts that good answers will be established for this and many other questions regarding the details of natural aging today. We may never know. The urge to investigate the demographics of aging will be swept away by the advent of rejuvenation therapies such as the senolytics presently under development. All natural variations in pace of aging and life expectancy will be buried beneath the size of the gains made possible through periodic repair of the cell and tissue damage that causes aging. The data will evaporate, and different concerns will occupy the scientific community of tomorrow. After all, how many members of today's scientific community spend any time on the demographics of smallpox in populations lacking treatment options? Few indeed. It will be the same for natural aging.
Mortality, life expectancy, and age-at-death are all strongly socially structured. Despite economic growth, welfare state provisions, modern medicine and a fundamental change in disease panorama, we find a negative social gradient in mortality generation after generation. Because education, occupation, and income all predict health and survival we should also expect such characteristics in the parental generation to predict the next generation's health prospects, resulting in "inheritance of longevity". It is possible, however, that this influence from previous generations is considerably broader than that working through the children's own education, occupation, and income. Variation in mortality risk within social groups is great. To understand "inheritance of longevity" we need a conceptual framework that also identifies those within-class influences.
Already in 1934 it was suggested that the first 15 years of life could determine your mortality risk during the entire lifecourse. Similarly, the so-called DOHaD (Developmental Origins of Health and Disease) theory suggests that early life experiences is an important determinant of adult health and disease. DOHaD theory has focused on specific aetiologies and influences, such as that of foetal growth restriction on blood pressure and circulatory disease. Another, earlier school of thinking argued for more general disease-causing mechanisms. Concepts like frailty, general susceptibility, or differential vulnerability refer to individual differences in the ability to survive hardship.
Demographic concepts like frailty, epidemiological ones like general susceptibility, and psychological ones like resilience all refer to the same real-life-phenomenon: a general rather than specific vulnerability to disease. Some have stressed its social roots, while others perhaps assumed it to have a more genetic basis. Resilience, in turn, may be related to both views. It could be thought of as the opposite extreme to susceptibility/frailty on the same underlying dimension. In this study, we argue that resilience is acquired early and maintained throughout life. Resilience should therefore influence the ability to survive up to a high age and be linked to longevity, as a number of studies indeed suggest.
"Inheritance of longevity" has been discussed at length in the literature. Its precise nature is somewhat elusive. Studying the entire Icelandic population, researchers concluded that longevity was inherited within families, probably because of shared genes. Other groups, looking at twin data, concluded that genetic influences on the lifespan were minimal before age 60 and only increase after that age. On the other hand, other work has rejected any idea that mortality in old age is genetically programmed. Consistent with that view, a Swedish study of men born in 1913, found that a number of social and behavioural factors measured at age 50, but not their parents' survival, predicted longevity.
Evolutionary theorists have debated whether there is any evolutionary pressure to promote survival into old age. Nevertheless, we observe a steady lifespan extension in modern societies, especially among women, partly based on falling mortality rates across their long post-reproductive period. That children tend to live longer than their parents is likely to be determined both by what experience parents brings to the next generation, and by the improved life circumstances of the children themselves in their childhood and adult life. The importance of genetic factors for longevity, we suggest, may lie in their interaction with other factors, perhaps especially if this interaction takes place at an early age.