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Can a lot more people live to one hundred and what if they did?
Accident Analysis and Prevention 61 (2013) 141–145
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Can a lot more people live to one hundred and what if they did? S. Jay Olshansky School of Public Health, University of Illinois at Chicago, United States
a r t i c l e
i n f o
Article history: Received 22 May 2012 Received in revised form 9 June 2013 Accepted 22 June 2013 Keywords: Aging Longevity Centenarians Mortality Life expectancy
a b s t r a c t In the 21st century humanity will witness unprecedented, increases in the number of older people, especially centenarians, in both the developed and developing world. From a public policy standpoint, the aging of our populations and the longer lives we experience will change, the fabric of our modern world – from the funding of age entitlement programs, to the rising cost of health care, to the new ways in which we transport ourselves in increasingly more urban environments. If it becomes possible for biomedical advances to manufacture a form of biological plasticity among the new generations reaching 85 and older in the future, much like that which now exists for the recent middle-aged classes of people aged 65–85, then the future of aging will be bright. If we fail to marshal resources to confront the biological processes of aging, then it is possible that the more destructive side of senescence will emerge. In this paper, I explore the various perspectives on the future course of longevity, examine the prospects for significant increases in the number of very old people – especially centenarians – and present a general view of the demographic aging of our changing society. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction The modern rise of exceptional longevity is one of society’s crowning achievements. After a long history of high birth rates and fluctuating death rates that led to a slow but steady rise in the duration of life over most of the last 150,000 years (McNeill, 1976), life expectancy increased rapidly in most of the developed world beginning in the middle of the 19th century (Omran, 1971), and for most other nations during the latter half of the 20th century. With the arrival of the 20th century humanity took what would appear to have been its firmest grip on the forces of mortality that limited life expectancy in the past, permitting the true longevity-related biology of our species to express itself for the first time in history. Children who would ordinarily have died from an infectious disease before age ten, now live decades longer – with many living well past the three score and ten promised in the Old Testament. Although the trade-off for longer life was the modern rise of fatal diseases such as cancer, heart disease, stroke, and Alzheimer’s disease, and chronic conditions including sensory impairments, arthritis and dementia, most would consider this to have been a fair trade. In today’s world we now see countries with life expectancies as high as 85 years for women and 78 years for men, and it would appear to some that the rise in longevity has no end is in sight (de Grey and Rae, 2007; Oeppen and Vaupel, 2002; Tuljapurkar et al., 2000; Vaupel, 2010; Wilmoth, 1998). A closely related and biologically fascinating phenomenon that is accompanying the road to
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higher life expectancy is the rapid increase in the number of centenarians – people who live for 100 years or more. The implications of these recent trends in longevity for a broad range of societal issues – including those involving transportation and safety – are self-evident. A question that has emerged in the modern study of human longevity is how much further the envelope of human survival can be extended and what the health of the older population will be in future decades? Is it possible that life expectancy in developed countries can continue its meteoric rise toward greater longevity like that experienced during most of the 20th century? Is it possible that the maximum lifespan of 122 years for humans, experienced by one person – Madame Jeanne Calment from France (Allard et al., 1998) – can be further extended, and would it have any scientific relevance if the world record for human longevity was broken? For that matter, is there any reason why humans cannot live to 200 years or more? Perhaps more important from a public policy perspective, can we expect the number of centenarians to continue to be the fastest growing segment of the age structure and for how long can this continue? Indeed, how might our world change if the number and proportion of centenarians continues its meteoric rise? 2. Can life expectancy at birth rise to 100 years or more? The question of how much higher human life expectancy can rise may be distilled down to three general views – each with their own logic and line of reasoning to which readers may be drawn depending on their leanings coming into the debate. One camp contends that yet-to-be-developed advances in biomedical technology
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and the anticipated emergence of nanotechnology are about to dramatically transform the landscape of human longevity – leading us down a pathway toward physical immortality and eternal youth. Under this scenario, not only will the number of centenarians rise rapidly – they will soon dominate the human age structure and old age as we know it today will disappear over time. A second camp believes that the historic rise in life expectancy will continue at its previous pace of about 2.5 years added per decade – leading to a life expectancy at birth of 100 years for birth cohorts to be born later in this century. Under this scenario, at least half of all the babies born today are expected to survive past age 100, and the total number of centenarians would rise rapidly for several generations – eventually leveling off at much higher numbers relative to today as the age structure stabilizes. The third camp contends that there are a number of identifiable and measurable biological, biodemographic, biomechanical, stochastic, and evolutionary reasons why life expectancy is unlikely to rise much beyond about 88 for women and 82 for men, but even this camp holds out hope that these limiting forces may eventually be breached somewhat by modern technology. Under this scenario, the number of centenarians would be expected to continue to rise rapidly until about the middle of this century, but then level off as the age structure stabilizes around low vital rates and a life expectancy at birth in the mid-80s for the entire population. I am a member of the third camp. My colleagues and I contend that there are a variety of biological and non-biological forces that influence and limit the duration of human life, and that in order to overcome these obstacles it will be necessary to first understand and then slow the biological processes of aging in enough people to influence a population-level statistic like life expectancy. We suggest that duration of life is currently limited by biodemographic forces, age-dependent changes in physiology, biomechanical constraints based on body design, the inevitable and strong stochastic component to biological aging, and the evidence that today’s younger generations are demonstrably less healthy than their predecessors (Seeman et al., 2010), but we remain optimistic that science can eventually find a way to slow aging in people (Olshansky et al., 2006). However, the byproduct of this effort would be an extension of healthy life – with only modest increases in life expectancy anticipated. A biodemographic constraint on duration of life has been established in the literature (Olshansky et al., 1990, 2001a,b). Mathematical features of the demographic measure of life expectancy reduce its sensitivity to declines in death rates as life expectancy rises. For example, when life expectancy at birth is 50 years, death rates at all ages must decline by approximately 4% in order for life expectancy to rise by one year to 51. Yet, when life expectancy at birth is 80, the same one-year rise in life expectancy, in this case to 81, requires that death rates decline by more than 9% at every age (Olshansky et al., 2001a). It is now well-established that as life expectancy at birth approaches or exceeds 80 years for any population, it becomes increasingly more difficult to raise this number further – even acknowledging the realistic expectation that death rates (especially at older ages) can continue to decline. The predictions by scientists in our camp (Olshansky et al., 1990) suggest that life expectancy at birth for females and males will not exceed 88 years and 82 years in any national population, respectively, until a breakthrough that slows aging in people is realized. Thus far, this prediction has proven accurate. The prediction that life expectancy at age 50 will not exceed 35 years (which applies to males and females combined) (Olshansky et al., 1990) has also proven accurate so far. Biomechanical forces that influence duration of life have been described in a literature that extends through the 20th century (Thompson, 1942; Morgan, 1994), with perhaps the most recent clear presentation of this line of reasoning by Olshansky et al.
(2001b). The basic idea behind biomechanical constraints on duration of life is that if the human body is viewed as a machine with moving parts that are equivalent to man-made pulleys, pumps, levers, and hinges, then it soon becomes evident that the human body is missing design features that would be required for extended use. Classic examples of this problem of life-limiting body design includes the wearing out of the hip and knee joints and the agedependent loss of bone, neurons, and muscle (especially that of the heart). The stochastic forces that influence duration of life have been described elsewhere (Kirkwood and Finch, 2000; Carnes et al., 2003). The importance of the random component to aging and duration of life should not be underestimated. When members of a species are raised under identical living conditions, even when the individuals are genetically identical, they do not all die on the same day as one might hypothesize – they experience a distribution of death that is nearly identical to that of genetically diverse members of a species raised under varying environmental conditions. The presence of random biological events having a significant influence on the processes of aging means that large increases in duration of life beyond those already achieved in low mortality populations, will be difficult to achieve. Furthermore, there is definitive evidence in the scientific literature demonstrating the presence of consistent age-dependent changes in the underlying physiology and reproductive biology of a variety of species (Carnes et al., 1996; Strehler and Mildvan, 1960). The presence of a consistent pathology of an aging phenotype across many species, including humans, suggests that most forms of life are subject to the equivalent of biological warranty periods that limit the duration of life. The evolutionary theory of senescence (senescence is defined as the biological changes that take place in our bodies with time that are associated with aging and disease expression) also provides a critical foundation for understanding why there are biological “limits” to the duration of life (Hamilton, 1966; Kirkwood, 1977; Medawar, 1952; Williams, 1957). According to evolution theory, the processes that contribute to senescence originate from forces that have nothing at all to do with aging. Evolution theory suggests that organisms that face high mortality pressures early in life (e.g. predation and infectious diseases), such as mice and other small animals, tend to go through puberty early in order to ensure that genes are passed onto the next generation. By contrast, species that face low extrinsic mortality tend to delay puberty and reproduction, one result of which is a longer lifespan. Thus, evolution theory suggests that the processes that contribute to senescence may very well be based on the presence of biological clocks, but those clocks exist not to regulate aging – but to orchestrate other life history events such as growth, development, and reproduction (Olshansky and Carnes, 2013). This means that the genetic forces that contribute to senescence are an inadvertent by-product of genetic programs that exist for other purposes and which are unrelated to aging. Evolution imposes biological “limits” on duration of life not because such limits evolved under the direct force of natural selection, but because selection neglects events that occur in the post-reproductive region of the lifespan. The weakness of our argument is that it rests largely on the assumption that existing knowledge and biomedical technologies can have only a limited benefit on life expectancy, and in order to go beyond these “limits” requires the very technologies that others believe are forthcoming. However, the fact is that no one knows with certainty how much more death rates can decline in the absence of futuristic life-extending biomedical breakthroughs. Current estimates of the effects of existing interventions on rising life expectancy rest heavily on the assumption that such benefits operate independent of one another – an assumption that is unrealistic when it comes to chronic diseases. Yet, the existence of dependence could yield either higher or lower effects of
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interventions on life expectancy – no one knows with certainty how this dependence of disease operates. Moreover, in Japan where the number one cause of death – heart disease – has nearly been eliminated, life expectancy has risen just beyond 85 years for women. Olshansky et al. (2001a) have suggested that life expectancy for females could rise to as high as 88 years in the absence of interventions that slow aging – a “barrier” that has yet to be breached by any national population.
3. Can a lot more people survive to age 100? The existence of biological and non-biological constraints on duration of life discussed in the previous section should not be confused with the evolving demographics of the age structure of our species (Olshansky et al., 1993). The modern rise of the centenarian population was the product of two events that occurred in the 20th century – the presence of relatively large cohorts from the early part of the century brought forth by migration and high fertility, and notable reductions in death rates at older ages in the latter decades of the century. The declines in middle and old-age mortality enabled those who might ordinarily have died in their 90s, to survive long enough to become a centenarian. Thus, there are more centenarians today not because of any documented deceleration of the processes of aging, but because of historical shifts in the age structure and recently improved survival prospects for nonagenerians and octogenarians. The modern rise in the number and proportion of centenarians is notable for a number of reasons. First, it is uncertain how frequently people might have lived that long in the past, but it was likely to have been an infrequent occurrance because of small population size and the very high risks of death at younger and middle ages – thus denying access to extreme old age for most. Some scientists have speculated that the occurrance of centenarians was as few as one per century prior to the 1800s (Juene, 1995), and the same author suggested that in Denmark where the birth and age records are very accurate for centuries in the past, there may have been no centenarians before the 19th century. Given that billions of people that lived prior to the 1800s, it seems likely that at least some lived to 100 years and more at some time in our history. Far more relevant is the meteoric rise in the absolute number of centenarians in the modern era. In 1950 there were an estimated 2300 centenarians in the U.S.; a figure that increased by more than 6-fold by 1980 to 15,000. By 1990 the number had risen to an estimated 28,000 (Kestenbaum, 1998). The Census Bureau reported 37,306 centenarians in 1990, but these numbers are known to be overestimates because of age reporting errors (Coale and Kisker, 1990; Kestenbaum, 1992). By 2000 the Census Bureau reported 50,000 centenarians. Under any circumstance, however, in the last half of the 20th century the absolute number of centenarians increased 20-fold. What will the future bring in terms of the sheer number of centenarians in the coming decades? Both the developed and developing world are about to experience a demographic explosion in the number of centenarians that will make their recent increases seem minor by comparison. For example, it has been estimated by the U.S. Census Bureau that by 2050 there could be as many as 834,000 people aged 100 and over. If death rates at older ages continue to decline even slightly more than is already anticipated by the Census Bureau in their middle range forecasts, this number could swell to over four million (Krach and Velkoff, 1999). Why is the centenarian population going to rise so much more rapidly in the next half century relative to the last 50 years? Because the perfect demographic and mortality conditions came together at one time. First, dramatic shifts in vital rates during the post World War II era yielded a combination of extremely large birth cohorts
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that were considerably larger than any previous cohorts in history, and this combined with rapid declines in death rates throughout the age structure. Although period life expectancy at birth continues to rise, recent evidence indicates that the first wave of the baby boom generation now aged 55–64 is facing levels of obesity and diabetes that are considerably worse than generations that passed through the same age range just 10 years earlier (NCHS, 2005; Seeman et al., 2010; Hulsegge et al., 2013). Thus, there are competing forces that could have a significant impact on the number of people who become centenarians in this century – large birth cohorts and currently declining old-age mortality contributing to their increase, and worsening health conditions among the baby boom generation and today’s children that could offset anticipated improvements in survival that are expected from advances in biomedical technology (Olshansky et al., 2005; NCHS, 2005; Reither et al., 2011). Finally, it is worth emphasizing that although the number of centenarians will invitably rise dramatically, this should not be interpreted to mean that life expectancy at birth will rise accordingly. These two demographic attributes of the population are related, but they are influenced largely by fundamentally different factors. The absolute number of centenarians is most heavily influenced by cohort size and age-specific probabilities of survival. By contrast, life expectancy is a product of a life table that takes into account death rates at all ages. This means it is possible that the number of centenarians can rise even during a time when life expectancy at birth declines, just as population size has been known to increase rapidly during an era of declining birth rates – a phenomenon that occurred in developed nations in the last three decades of the 20th century. The demographic variables of population size and shifts in the age structure, including the rise of the centenarian population, are largely a product of the momentum for their occurrance that is built into the age structure from demographic events that occurred decades earlier.
4. The demographic and health consequences of a rise in the number of centenarians Having documented the historic rise in the number of centenarians and their inevitable rapid rise in this century, readers need to be aware of the unique life history dynamics of this subgroup of the population and how this is likely to change. People reaching the century mark today were all born in the late 19th or early 20th centuries; they were too young to participate in the first World War but came of age during the 1918 global influenza pandemic; they started their families during the Great Depression and had some of the highest rates of childlessness ever recorded in the U.S. (Himes, 2005); they were too old to participate in World War II and the Korean War; and they began retiring at the start of the Vietnam era of the 1970s. Today’s centenarians are extraordinarily unique in every way, and therein lies a dilemma not just for scientists who study them, but for future generations that may live that long in the coming decades. The conventional wisdom among many gerontologists until fairly recently was that senescence is always accompanied by declines in physical and mental functioning. This view was supported by early work in the field in which it was demonstrated that a wide array of physiological attributes of the human body decline with age (Strehler and Mildvan, 1960). However, researchers who carefully evaluated modern elderly cohorts discovered that although changes in both body and mind accompany the passage of time, this did not always lead to the expected deficits in function (Perls et al., 2000; Rowe and Kahn, 1987). To the contrary, the extreme elderly showed a remarkable resilience and ability to adapt to the changes associated with growing older – with
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many centenarians retaining their functioning at surprisingly high levels. In fact, a recently published book by the World Economic Forum (2012) suggests that older cohorts in the future should be thought of as a valuable natural resource that should be nurtured. The dilemma is that the current generation of centenarians has been highly selected by the unique life conditions in which they lived. These unique long-lived individuals largely represent the hardiest survivors among a large cohort that began life in a world in which life expectancy for the population was less than half their current age. Since the vast majority of their birth cohort has already died, in a way, today’s centenarians are an abberation of the usual aging and death pattern experienced by most people. It is tempting to hold up this unique subgroup as an example of what is theoretically achievable for all, but this would be equivalent to declaring that because a handful of exceptional athletes can run one mile in four minutes, everyone should therefore be able to do so. Because humans are genetically heterogeneous, the rest of the population cannot as yet experience the unique biological attributes of exceptional survivors. Nevertheless, it is this exceptional quality that makes today’s centenarians such an important group of people to study in terms of both lifestyle and genetics. Indeed, the secret to their exceptional survival may very well lie within their genome and relatively rare and complex genetic and environmental/behavioral interactions – an area of research that may eventually lead to the discovery of the alleles that confer extended protection from the fatal diseases that prove lethal to the rest of the population at earlier ages (Geesaman et al., 2003; Kirkland, 2013). The delemma that researchers in the field are about to confront is that with further anticipated declines in old-age mortality resulting from the more efficient treatment of the pathology of disease, entrance requirements into the unique club of exceptional survivors will no longer be restricted only to those who possess the right genes. With medical technology continuing to manufacture survival time (Olshansky et al., 1998), people who would never ordinarily have had an opportunity to live into their 80s, 90s, or for 100 years, are going to do so with increasing frequency. By relaxing the selection pressures on exceptional survival, biological heterogeneity at the extreme end of the age structure will inevitably increase. What this means for the future health of elderly cohorts is uncertain – but one thing is for sure; people who live past the century mark in the coming decades are likely to look and act quite differently than the pioneers who did so in the last half century. On the one hand we may witness a sharp rise in the number of exceptionally healthy and unusually vibrant extreme elderly as those who combine the right genes with a lifelong history of proper diet and exercise reap the benefits of their lifestyle choices and genetic good fortune. On the other hand, we may also see a sharp rise in the prevalence of extreme frailty, and possibly even new or rarely observed health conditions appearing among centenarians in the future as their extended survival enables the bodies of these individuals to express disabling and lethal conditions that would not otherwise have been possible to see. If both scenarios occur simultaneously, as this researcher expects, then humanity is about to witness both the best and the worst of what exceptional survival has to offer.
now held by Jeanne Calment from France at 122 years, will be broken in this century – an event that is likely to occur for no other reason than the very large birth cohorts approaching older ages. However, as interesting as this may be to some who like to follow the extremes of any event, breaking the longevity record will hold little meaning to the biological reality of exceptional survival in a genetically heterogeneous population. What is far more interesting and important from a public policy standpoint is how the modern rise of an entirely new and different elderly cohort, composed of those aged 85–100, and the extreme elderly composed of those reaching 100 and over, may change the fabric of our modern world. If it becomes possible for biomedical advances to manufacture a form of biological plasticity among the new generations reaching 85 and older, much like that which now exists for the recent middle-aged classes of people aged 65–85, then the future of aging will be very bright. If we fail to marshall our resources to confront the biological processes of aging, then it is possible that the more destructive side of senescence will emerge. Scientists have suggested that efforts to slow aging are not only plausible in the coming decades (Kirkland, 2013; Miller, 2009; Sierra et al., 2008), but they should be aggressively pursued as the most efficient method of improving health and quality of life among future cohorts of older persons (Butler et al., 2008; Olshansky et al., 2006, 2012). The implications of these ongoing and future demographic shifts for the changing mobility needs of future cohorts of older persons should be apparent. With a growing older population we will see the extremes of healthy life and disability begin to emerge. Of greatest concern will be the inevitable increase in the prevalence of populations with increasingly more challenging physical disabilities. The modern rise of adult-onset obesity and aging-related frailty will place unique demands on public transportation systems – especially our current reliance on automobiles. There is reason to believe the basic structure of our cars will need to change to accommodate these new physical realities, and public transportation will need to evolve to ensure that frailer segments of the population can have easy access to and egress from buses and trains and other forms of transportation. A new consortium of scientists in the field of aging and representatives from automobile manufacturers (among other private sectors of society) have begun to meet to discuss the unique impact of a rising older population on transportation systems in the U.S. – a collaboration that will no doubt yield valuable interventions to improve the mobility of current and future cohorts of older persons (Society, 2030). The pathway to exceptional survival has been lit by the longevity pioneers who lived through the last century, but it is the baby boomers who will drive this road in the first half of this century. This post World War II generation has already changed patterns of work, marriage, retirement, and income, and now the longevity revolution is about to wreak its own special havoc on all of these social and economic forces. Perhaps the most important legacy to be left behind by this generation will not be a future that has been mortgaged by self-indulgent actions that harmed the environment, but by an everlasting gift to future generations (even if intended for themselves first) of a long life accompanied by good health.
References 5. Conclusions In the 21st century it is inevitable that there will be unprecedented increases in the absolute number of centenarians in both the developed and developing world – regardless of how much life expectancy at birth or at older ages changes in the coming decades. It may well be the case that the world record for human longevity,
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Can a lot more people live to one hundred and what if they did? S. Jay Olshansky School of Public Health, University of Illinois at Chicago, United States
a r t i c l e
i n f o
Article history: Received 22 May 2012 Received in revised form 9 June 2013 Accepted 22 June 2013 Keywords: Aging Longevity Centenarians Mortality Life expectancy
a b s t r a c t In the 21st century humanity will witness unprecedented, increases in the number of older people, especially centenarians, in both the developed and developing world. From a public policy standpoint, the aging of our populations and the longer lives we experience will change, the fabric of our modern world – from the funding of age entitlement programs, to the rising cost of health care, to the new ways in which we transport ourselves in increasingly more urban environments. If it becomes possible for biomedical advances to manufacture a form of biological plasticity among the new generations reaching 85 and older in the future, much like that which now exists for the recent middle-aged classes of people aged 65–85, then the future of aging will be bright. If we fail to marshal resources to confront the biological processes of aging, then it is possible that the more destructive side of senescence will emerge. In this paper, I explore the various perspectives on the future course of longevity, examine the prospects for significant increases in the number of very old people – especially centenarians – and present a general view of the demographic aging of our changing society. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction The modern rise of exceptional longevity is one of society’s crowning achievements. After a long history of high birth rates and fluctuating death rates that led to a slow but steady rise in the duration of life over most of the last 150,000 years (McNeill, 1976), life expectancy increased rapidly in most of the developed world beginning in the middle of the 19th century (Omran, 1971), and for most other nations during the latter half of the 20th century. With the arrival of the 20th century humanity took what would appear to have been its firmest grip on the forces of mortality that limited life expectancy in the past, permitting the true longevity-related biology of our species to express itself for the first time in history. Children who would ordinarily have died from an infectious disease before age ten, now live decades longer – with many living well past the three score and ten promised in the Old Testament. Although the trade-off for longer life was the modern rise of fatal diseases such as cancer, heart disease, stroke, and Alzheimer’s disease, and chronic conditions including sensory impairments, arthritis and dementia, most would consider this to have been a fair trade. In today’s world we now see countries with life expectancies as high as 85 years for women and 78 years for men, and it would appear to some that the rise in longevity has no end is in sight (de Grey and Rae, 2007; Oeppen and Vaupel, 2002; Tuljapurkar et al., 2000; Vaupel, 2010; Wilmoth, 1998). A closely related and biologically fascinating phenomenon that is accompanying the road to
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higher life expectancy is the rapid increase in the number of centenarians – people who live for 100 years or more. The implications of these recent trends in longevity for a broad range of societal issues – including those involving transportation and safety – are self-evident. A question that has emerged in the modern study of human longevity is how much further the envelope of human survival can be extended and what the health of the older population will be in future decades? Is it possible that life expectancy in developed countries can continue its meteoric rise toward greater longevity like that experienced during most of the 20th century? Is it possible that the maximum lifespan of 122 years for humans, experienced by one person – Madame Jeanne Calment from France (Allard et al., 1998) – can be further extended, and would it have any scientific relevance if the world record for human longevity was broken? For that matter, is there any reason why humans cannot live to 200 years or more? Perhaps more important from a public policy perspective, can we expect the number of centenarians to continue to be the fastest growing segment of the age structure and for how long can this continue? Indeed, how might our world change if the number and proportion of centenarians continues its meteoric rise? 2. Can life expectancy at birth rise to 100 years or more? The question of how much higher human life expectancy can rise may be distilled down to three general views – each with their own logic and line of reasoning to which readers may be drawn depending on their leanings coming into the debate. One camp contends that yet-to-be-developed advances in biomedical technology
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and the anticipated emergence of nanotechnology are about to dramatically transform the landscape of human longevity – leading us down a pathway toward physical immortality and eternal youth. Under this scenario, not only will the number of centenarians rise rapidly – they will soon dominate the human age structure and old age as we know it today will disappear over time. A second camp believes that the historic rise in life expectancy will continue at its previous pace of about 2.5 years added per decade – leading to a life expectancy at birth of 100 years for birth cohorts to be born later in this century. Under this scenario, at least half of all the babies born today are expected to survive past age 100, and the total number of centenarians would rise rapidly for several generations – eventually leveling off at much higher numbers relative to today as the age structure stabilizes. The third camp contends that there are a number of identifiable and measurable biological, biodemographic, biomechanical, stochastic, and evolutionary reasons why life expectancy is unlikely to rise much beyond about 88 for women and 82 for men, but even this camp holds out hope that these limiting forces may eventually be breached somewhat by modern technology. Under this scenario, the number of centenarians would be expected to continue to rise rapidly until about the middle of this century, but then level off as the age structure stabilizes around low vital rates and a life expectancy at birth in the mid-80s for the entire population. I am a member of the third camp. My colleagues and I contend that there are a variety of biological and non-biological forces that influence and limit the duration of human life, and that in order to overcome these obstacles it will be necessary to first understand and then slow the biological processes of aging in enough people to influence a population-level statistic like life expectancy. We suggest that duration of life is currently limited by biodemographic forces, age-dependent changes in physiology, biomechanical constraints based on body design, the inevitable and strong stochastic component to biological aging, and the evidence that today’s younger generations are demonstrably less healthy than their predecessors (Seeman et al., 2010), but we remain optimistic that science can eventually find a way to slow aging in people (Olshansky et al., 2006). However, the byproduct of this effort would be an extension of healthy life – with only modest increases in life expectancy anticipated. A biodemographic constraint on duration of life has been established in the literature (Olshansky et al., 1990, 2001a,b). Mathematical features of the demographic measure of life expectancy reduce its sensitivity to declines in death rates as life expectancy rises. For example, when life expectancy at birth is 50 years, death rates at all ages must decline by approximately 4% in order for life expectancy to rise by one year to 51. Yet, when life expectancy at birth is 80, the same one-year rise in life expectancy, in this case to 81, requires that death rates decline by more than 9% at every age (Olshansky et al., 2001a). It is now well-established that as life expectancy at birth approaches or exceeds 80 years for any population, it becomes increasingly more difficult to raise this number further – even acknowledging the realistic expectation that death rates (especially at older ages) can continue to decline. The predictions by scientists in our camp (Olshansky et al., 1990) suggest that life expectancy at birth for females and males will not exceed 88 years and 82 years in any national population, respectively, until a breakthrough that slows aging in people is realized. Thus far, this prediction has proven accurate. The prediction that life expectancy at age 50 will not exceed 35 years (which applies to males and females combined) (Olshansky et al., 1990) has also proven accurate so far. Biomechanical forces that influence duration of life have been described in a literature that extends through the 20th century (Thompson, 1942; Morgan, 1994), with perhaps the most recent clear presentation of this line of reasoning by Olshansky et al.
(2001b). The basic idea behind biomechanical constraints on duration of life is that if the human body is viewed as a machine with moving parts that are equivalent to man-made pulleys, pumps, levers, and hinges, then it soon becomes evident that the human body is missing design features that would be required for extended use. Classic examples of this problem of life-limiting body design includes the wearing out of the hip and knee joints and the agedependent loss of bone, neurons, and muscle (especially that of the heart). The stochastic forces that influence duration of life have been described elsewhere (Kirkwood and Finch, 2000; Carnes et al., 2003). The importance of the random component to aging and duration of life should not be underestimated. When members of a species are raised under identical living conditions, even when the individuals are genetically identical, they do not all die on the same day as one might hypothesize – they experience a distribution of death that is nearly identical to that of genetically diverse members of a species raised under varying environmental conditions. The presence of random biological events having a significant influence on the processes of aging means that large increases in duration of life beyond those already achieved in low mortality populations, will be difficult to achieve. Furthermore, there is definitive evidence in the scientific literature demonstrating the presence of consistent age-dependent changes in the underlying physiology and reproductive biology of a variety of species (Carnes et al., 1996; Strehler and Mildvan, 1960). The presence of a consistent pathology of an aging phenotype across many species, including humans, suggests that most forms of life are subject to the equivalent of biological warranty periods that limit the duration of life. The evolutionary theory of senescence (senescence is defined as the biological changes that take place in our bodies with time that are associated with aging and disease expression) also provides a critical foundation for understanding why there are biological “limits” to the duration of life (Hamilton, 1966; Kirkwood, 1977; Medawar, 1952; Williams, 1957). According to evolution theory, the processes that contribute to senescence originate from forces that have nothing at all to do with aging. Evolution theory suggests that organisms that face high mortality pressures early in life (e.g. predation and infectious diseases), such as mice and other small animals, tend to go through puberty early in order to ensure that genes are passed onto the next generation. By contrast, species that face low extrinsic mortality tend to delay puberty and reproduction, one result of which is a longer lifespan. Thus, evolution theory suggests that the processes that contribute to senescence may very well be based on the presence of biological clocks, but those clocks exist not to regulate aging – but to orchestrate other life history events such as growth, development, and reproduction (Olshansky and Carnes, 2013). This means that the genetic forces that contribute to senescence are an inadvertent by-product of genetic programs that exist for other purposes and which are unrelated to aging. Evolution imposes biological “limits” on duration of life not because such limits evolved under the direct force of natural selection, but because selection neglects events that occur in the post-reproductive region of the lifespan. The weakness of our argument is that it rests largely on the assumption that existing knowledge and biomedical technologies can have only a limited benefit on life expectancy, and in order to go beyond these “limits” requires the very technologies that others believe are forthcoming. However, the fact is that no one knows with certainty how much more death rates can decline in the absence of futuristic life-extending biomedical breakthroughs. Current estimates of the effects of existing interventions on rising life expectancy rest heavily on the assumption that such benefits operate independent of one another – an assumption that is unrealistic when it comes to chronic diseases. Yet, the existence of dependence could yield either higher or lower effects of
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interventions on life expectancy – no one knows with certainty how this dependence of disease operates. Moreover, in Japan where the number one cause of death – heart disease – has nearly been eliminated, life expectancy has risen just beyond 85 years for women. Olshansky et al. (2001a) have suggested that life expectancy for females could rise to as high as 88 years in the absence of interventions that slow aging – a “barrier” that has yet to be breached by any national population.
3. Can a lot more people survive to age 100? The existence of biological and non-biological constraints on duration of life discussed in the previous section should not be confused with the evolving demographics of the age structure of our species (Olshansky et al., 1993). The modern rise of the centenarian population was the product of two events that occurred in the 20th century – the presence of relatively large cohorts from the early part of the century brought forth by migration and high fertility, and notable reductions in death rates at older ages in the latter decades of the century. The declines in middle and old-age mortality enabled those who might ordinarily have died in their 90s, to survive long enough to become a centenarian. Thus, there are more centenarians today not because of any documented deceleration of the processes of aging, but because of historical shifts in the age structure and recently improved survival prospects for nonagenerians and octogenarians. The modern rise in the number and proportion of centenarians is notable for a number of reasons. First, it is uncertain how frequently people might have lived that long in the past, but it was likely to have been an infrequent occurrance because of small population size and the very high risks of death at younger and middle ages – thus denying access to extreme old age for most. Some scientists have speculated that the occurrance of centenarians was as few as one per century prior to the 1800s (Juene, 1995), and the same author suggested that in Denmark where the birth and age records are very accurate for centuries in the past, there may have been no centenarians before the 19th century. Given that billions of people that lived prior to the 1800s, it seems likely that at least some lived to 100 years and more at some time in our history. Far more relevant is the meteoric rise in the absolute number of centenarians in the modern era. In 1950 there were an estimated 2300 centenarians in the U.S.; a figure that increased by more than 6-fold by 1980 to 15,000. By 1990 the number had risen to an estimated 28,000 (Kestenbaum, 1998). The Census Bureau reported 37,306 centenarians in 1990, but these numbers are known to be overestimates because of age reporting errors (Coale and Kisker, 1990; Kestenbaum, 1992). By 2000 the Census Bureau reported 50,000 centenarians. Under any circumstance, however, in the last half of the 20th century the absolute number of centenarians increased 20-fold. What will the future bring in terms of the sheer number of centenarians in the coming decades? Both the developed and developing world are about to experience a demographic explosion in the number of centenarians that will make their recent increases seem minor by comparison. For example, it has been estimated by the U.S. Census Bureau that by 2050 there could be as many as 834,000 people aged 100 and over. If death rates at older ages continue to decline even slightly more than is already anticipated by the Census Bureau in their middle range forecasts, this number could swell to over four million (Krach and Velkoff, 1999). Why is the centenarian population going to rise so much more rapidly in the next half century relative to the last 50 years? Because the perfect demographic and mortality conditions came together at one time. First, dramatic shifts in vital rates during the post World War II era yielded a combination of extremely large birth cohorts
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that were considerably larger than any previous cohorts in history, and this combined with rapid declines in death rates throughout the age structure. Although period life expectancy at birth continues to rise, recent evidence indicates that the first wave of the baby boom generation now aged 55–64 is facing levels of obesity and diabetes that are considerably worse than generations that passed through the same age range just 10 years earlier (NCHS, 2005; Seeman et al., 2010; Hulsegge et al., 2013). Thus, there are competing forces that could have a significant impact on the number of people who become centenarians in this century – large birth cohorts and currently declining old-age mortality contributing to their increase, and worsening health conditions among the baby boom generation and today’s children that could offset anticipated improvements in survival that are expected from advances in biomedical technology (Olshansky et al., 2005; NCHS, 2005; Reither et al., 2011). Finally, it is worth emphasizing that although the number of centenarians will invitably rise dramatically, this should not be interpreted to mean that life expectancy at birth will rise accordingly. These two demographic attributes of the population are related, but they are influenced largely by fundamentally different factors. The absolute number of centenarians is most heavily influenced by cohort size and age-specific probabilities of survival. By contrast, life expectancy is a product of a life table that takes into account death rates at all ages. This means it is possible that the number of centenarians can rise even during a time when life expectancy at birth declines, just as population size has been known to increase rapidly during an era of declining birth rates – a phenomenon that occurred in developed nations in the last three decades of the 20th century. The demographic variables of population size and shifts in the age structure, including the rise of the centenarian population, are largely a product of the momentum for their occurrance that is built into the age structure from demographic events that occurred decades earlier.
4. The demographic and health consequences of a rise in the number of centenarians Having documented the historic rise in the number of centenarians and their inevitable rapid rise in this century, readers need to be aware of the unique life history dynamics of this subgroup of the population and how this is likely to change. People reaching the century mark today were all born in the late 19th or early 20th centuries; they were too young to participate in the first World War but came of age during the 1918 global influenza pandemic; they started their families during the Great Depression and had some of the highest rates of childlessness ever recorded in the U.S. (Himes, 2005); they were too old to participate in World War II and the Korean War; and they began retiring at the start of the Vietnam era of the 1970s. Today’s centenarians are extraordinarily unique in every way, and therein lies a dilemma not just for scientists who study them, but for future generations that may live that long in the coming decades. The conventional wisdom among many gerontologists until fairly recently was that senescence is always accompanied by declines in physical and mental functioning. This view was supported by early work in the field in which it was demonstrated that a wide array of physiological attributes of the human body decline with age (Strehler and Mildvan, 1960). However, researchers who carefully evaluated modern elderly cohorts discovered that although changes in both body and mind accompany the passage of time, this did not always lead to the expected deficits in function (Perls et al., 2000; Rowe and Kahn, 1987). To the contrary, the extreme elderly showed a remarkable resilience and ability to adapt to the changes associated with growing older – with
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many centenarians retaining their functioning at surprisingly high levels. In fact, a recently published book by the World Economic Forum (2012) suggests that older cohorts in the future should be thought of as a valuable natural resource that should be nurtured. The dilemma is that the current generation of centenarians has been highly selected by the unique life conditions in which they lived. These unique long-lived individuals largely represent the hardiest survivors among a large cohort that began life in a world in which life expectancy for the population was less than half their current age. Since the vast majority of their birth cohort has already died, in a way, today’s centenarians are an abberation of the usual aging and death pattern experienced by most people. It is tempting to hold up this unique subgroup as an example of what is theoretically achievable for all, but this would be equivalent to declaring that because a handful of exceptional athletes can run one mile in four minutes, everyone should therefore be able to do so. Because humans are genetically heterogeneous, the rest of the population cannot as yet experience the unique biological attributes of exceptional survivors. Nevertheless, it is this exceptional quality that makes today’s centenarians such an important group of people to study in terms of both lifestyle and genetics. Indeed, the secret to their exceptional survival may very well lie within their genome and relatively rare and complex genetic and environmental/behavioral interactions – an area of research that may eventually lead to the discovery of the alleles that confer extended protection from the fatal diseases that prove lethal to the rest of the population at earlier ages (Geesaman et al., 2003; Kirkland, 2013). The delemma that researchers in the field are about to confront is that with further anticipated declines in old-age mortality resulting from the more efficient treatment of the pathology of disease, entrance requirements into the unique club of exceptional survivors will no longer be restricted only to those who possess the right genes. With medical technology continuing to manufacture survival time (Olshansky et al., 1998), people who would never ordinarily have had an opportunity to live into their 80s, 90s, or for 100 years, are going to do so with increasing frequency. By relaxing the selection pressures on exceptional survival, biological heterogeneity at the extreme end of the age structure will inevitably increase. What this means for the future health of elderly cohorts is uncertain – but one thing is for sure; people who live past the century mark in the coming decades are likely to look and act quite differently than the pioneers who did so in the last half century. On the one hand we may witness a sharp rise in the number of exceptionally healthy and unusually vibrant extreme elderly as those who combine the right genes with a lifelong history of proper diet and exercise reap the benefits of their lifestyle choices and genetic good fortune. On the other hand, we may also see a sharp rise in the prevalence of extreme frailty, and possibly even new or rarely observed health conditions appearing among centenarians in the future as their extended survival enables the bodies of these individuals to express disabling and lethal conditions that would not otherwise have been possible to see. If both scenarios occur simultaneously, as this researcher expects, then humanity is about to witness both the best and the worst of what exceptional survival has to offer.
now held by Jeanne Calment from France at 122 years, will be broken in this century – an event that is likely to occur for no other reason than the very large birth cohorts approaching older ages. However, as interesting as this may be to some who like to follow the extremes of any event, breaking the longevity record will hold little meaning to the biological reality of exceptional survival in a genetically heterogeneous population. What is far more interesting and important from a public policy standpoint is how the modern rise of an entirely new and different elderly cohort, composed of those aged 85–100, and the extreme elderly composed of those reaching 100 and over, may change the fabric of our modern world. If it becomes possible for biomedical advances to manufacture a form of biological plasticity among the new generations reaching 85 and older, much like that which now exists for the recent middle-aged classes of people aged 65–85, then the future of aging will be very bright. If we fail to marshall our resources to confront the biological processes of aging, then it is possible that the more destructive side of senescence will emerge. Scientists have suggested that efforts to slow aging are not only plausible in the coming decades (Kirkland, 2013; Miller, 2009; Sierra et al., 2008), but they should be aggressively pursued as the most efficient method of improving health and quality of life among future cohorts of older persons (Butler et al., 2008; Olshansky et al., 2006, 2012). The implications of these ongoing and future demographic shifts for the changing mobility needs of future cohorts of older persons should be apparent. With a growing older population we will see the extremes of healthy life and disability begin to emerge. Of greatest concern will be the inevitable increase in the prevalence of populations with increasingly more challenging physical disabilities. The modern rise of adult-onset obesity and aging-related frailty will place unique demands on public transportation systems – especially our current reliance on automobiles. There is reason to believe the basic structure of our cars will need to change to accommodate these new physical realities, and public transportation will need to evolve to ensure that frailer segments of the population can have easy access to and egress from buses and trains and other forms of transportation. A new consortium of scientists in the field of aging and representatives from automobile manufacturers (among other private sectors of society) have begun to meet to discuss the unique impact of a rising older population on transportation systems in the U.S. – a collaboration that will no doubt yield valuable interventions to improve the mobility of current and future cohorts of older persons (Society, 2030). The pathway to exceptional survival has been lit by the longevity pioneers who lived through the last century, but it is the baby boomers who will drive this road in the first half of this century. This post World War II generation has already changed patterns of work, marriage, retirement, and income, and now the longevity revolution is about to wreak its own special havoc on all of these social and economic forces. Perhaps the most important legacy to be left behind by this generation will not be a future that has been mortgaged by self-indulgent actions that harmed the environment, but by an everlasting gift to future generations (even if intended for themselves first) of a long life accompanied by good health.
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