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Forecasting continuously increasing life expectancy: What implications?
Ageing Research Reviews 11 (2012) 325–328
Contents lists available at SciVerse ScienceDirect
Ageing Research Reviews journal homepage: www.elsevier.com/locate/arr
Review
Forecasting continuously increasing life expectancy: What implications? Éric Le Bourg ∗ Université Paul-Sabatier, Centre de Recherche sur la Cognition Animale, UMR CNRS 5169, 118 route de Narbonne, F-31062 Toulouse Cedex 9, France
a r t i c l e
i n f o
Article history: Received 14 December 2011 Received in revised form 16 January 2012 Accepted 19 January 2012 Available online 1 February 2012 Keywords: Human beings Median lifespan Maximal lifespan Forecasts
a b s t r a c t It has been proposed that life expectancy could linearly increase in the next decades and that median longevity of the youngest birth cohorts could reach 105 years or more. These forecasts have been criticized but it seems that their implications for future maximal lifespan (i.e. the lifespan of the last survivors) have not been considered. These implications make these forecasts untenable and it is less risky to hypothesize that life expectancy and maximal lifespan will reach an asymptotic limit in some decades from now. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Forecasting human longevity is a risky business and there is a long-lasting debate on the existence of limits to both life expectancy at birth (e.g. Olshansky et al., 1990) and maximal lifespan (e.g. Kirkwood, 1997), i.e. the lifespan of the last survivors, which is currently ca. 115 years (if we except the record-woman Jeanne Calment, who died at 122 years of age, Fig. 1). Some demographers have hypothetized that life expectancy could reach 100 years or more in the youngest birth cohorts, and these ideas have attracted much interest from mass media, as well as provoking reactions of other authors (e.g. Olshansky and Carnes, 2001; Carnes et al., 2003). These last authors have rightly relied on biological evidence to refute the idea of an always increasing lifespan. The purpose of this article is to show that, beyond these biological arguments, this hypothesis of a very long life expectancy is logically untenable because of implications regarding maximal lifespan and evolutionary biology.
observed each year among all countries, i.e. the “record female life expectancy” has increased in a linear way since 1840, the same result being also observed for males. Going a step further, they wrote that “one reasonable scenario would be that this trend will continue in coming decades. If so, record life expectancy will reach 100 in about six decades”. Christensen et al. (2009) wrote that “most babies born since 2000 in countries with long-lived residents will celebrate their 100th birthdays if the present yearly growth in life expectancy continues through the 21st century”. Table 1 in the last article thus showed that the median lifespan of French babies (both sexes together) born in 2000 could reach 102 years and that of the 2007 birth cohort 104 years, due to a 0.2 year life expectancy increase each year. As this table and the text of the article do not indicate a time limit, it can be assumed that babies of younger cohorts could even live longer, even if this hypothesis is not formally assessed (or disclaimed) by the authors. Indeed, Table 1 of the article is expecting a 107 years median lifespan for the 2007 Japanese birth cohort and thus, due to the longevity gap between genders, probably a still higher lifespan for Japanese women.
2. The forecasts for very high life expectancies. . . In 1997, Vaupel (1997a) stressed that his “best guess. . . is that half of the babies born in France this year (1996) will reach age 95 and that most of the girls will reach age 100”. A few years later, Oeppen and Vaupel (2002) stressed that previous attempts to predict life expectancy had all been surpassed by observation. They displayed a graph showing that the highest women life expectancy
∗ Fax: +33 5 61 55 61 54. E-mail address: [email protected] 1568-1637/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.arr.2012.01.002
3. .. . . and their implications for maximal lifespan What is the hypothesis of previously cited demographers regarding future maximal lifespan? Obviously, if life expectancy at birth would increase beyond 105 years, it would be very close to the current maximal lifespan but, oddly enough, these authors did not consider the evolution of maximal lifespan in their articles. However, in the box 1 of his article, Vaupel (2010) stressed that “the process of senescence is being delayed rather than being decelerated”. Consistent with this hypothesis, Figure 2 in Vaupel (2010) showed that mortality rates of most longevous sub-cohorts do not
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that maximal lifespan would not increase if median lifespan would reach, e.g. 107 years of age. To go a step further, if median lifespan would increase up to, say, 110 or 115 years of age, 50% of the birth cohort would die in a few months if maximal lifespan would not increase. Thus, the hypothesis of no maximal lifespan increase if life expectancy would reach 105 and beyond is a logical dead-end.
Maximal recorded longevity (years)
125
120
3.2. Second hypothesis: maximal lifespan increase 115
110
105 1950
1960
1970
1980
1990
2000
2010
Year of death Fig. 1. Maximal recorded human longevity from 1955 to 2010. Data are extracted from table CCCC on the Gerontology Research Group website (www.grg.org) and dubious cases of the table are not included in the figure. Data on maximal lifespan are also available from the International Database on Longevity (www.supercentenarians.org). The first confirmed super-centenarian (i.e. a person living longer than 110 years) died in 1932 at 111 years of age and probably no other super-centenarian existed before this time (Vaupel, 1997b; Jeune and Kannisto, 1997; Wilmoth et al., 2000). The highest maximal lifespan ever observed is that of Jeanne Calment, who died at 122 years of age in 1997.
differ from other cohorts by their slope and, as these longevous subcohorts age at the same rate than less longevous ones, that their mortality rates can be very low at oldest ages. Therefore, it can be probably deduced from the Vaupel’s (2010) hypothesis of the “delay not deceleration” of the aging process that maximal lifespan is expected to increase when median lifespan increases. Nevertheless, let us examine the two hypotheses regarding maximal lifespan changes: no increase or increase. 3.1. First hypothesis: no maximal lifespan increase In this hypothesis, maximal lifespan would thus remain around ca. 115 years, i.e. the maximal lifespan observed for ca. one century (Fig. 1). It follows, since median lifespan would reach e.g. 107 years for the 2007 Japanese birth cohort (Christensen et al., 2009), that 50% of this cohort would die between 107 and ca. 115 years of age. Therefore, there would be a strong compression of mortality in the next decades: as the modal age of death would increase more quickly than maximal lifespan, the variability above this modal age would decrease. Such a compression of mortality has been observed in developed countries during the last century (e.g. Wilmoth, 2000; Thatcher et al., 2010), but the variability above the modal age of death seems to have stopped to decrease in recent years in some countries (Switzerland, Japan, France or Italy, see Cheung et al., 2009). Is it biologically plausible that mortality rates would be very low up to, say, 107 years of age and, after that, would suddenly increase up to reach 50% each year, i.e. the death rates currently observed above 110 years of age (Gampe, 2010; Thatcher, 2010)? This hypothesis seems untenable because the variability of lifespan would be very low, 50% of the cohort dying in a very few years, while it is known that there is a high variability of lifespans even in inbred strains and in monozygotic human twins (Finch and Kirkwood, 2000; Kirkwood et al., 2005). It is thus difficult to accept that genetically heterogeneous people with different living conditions could die in a very narrow time span. It seems thus impossible
It thus seems that maximal lifespan should increase if median lifespan would reach, e.g. 107 years of age. The hypothesis of a maximal lifespan increase by a right-shifting of the survival curve, i.e. by maintaining the rectangularization of the survival curve, has been studied by Vallin and Caselli (1997): if life expectancy would reach 105 years of age maximal lifespan would be around 140 years (145 years if life expectancy were 110). In other words, maximal lifespan should increase by ca. 25 years in the next decades, if we consider that the most longevous supercentenarians die at ca. 115 years of age, or by ca. 20 years, if we consider that the current record of 122 years is more representative of the actual maximal lifespan. In any case, Fig. 1 shows that maximal lifespan is rather steady for decades (if we except three exceptional cases of 119 and 122 years of age), despite huge increases of life expectancy during this period, and it is difficult to admit that maximal lifespan could suddenly increase to such a large extent. 3.3. Conclusions: maximal lifespan can neither increase nor remain steady if life expectancy is above 105 It seems difficult to support the idea that maximal lifespan would not change if life expectancy would be well above 100. Anyway, predicting that maximal lifespan would increase implies to explain how reaching so high maximal values. Therefore, we are let with the strong hypothesis that median lifespan could be well above 100 for the youngest birth cohorts (and maybe more for those not still born), but with the puzzling conclusion that maximal lifespan could not remain the same as today, nor increase unless to accept that it could be much higher than the maximal lifespans observed for decades. Indeed, when hypotheses predict substantial changes (and predicting that maximal lifespan could reach much higher values than today is a substantial change), “the burden of the proof lies with those who predict sharp deviations from past trends”, as stressed by Wilmoth (2000). Therefore, it seems that the hypothesis of a continuous linear increase of life expectancy in the next decades up to reach, e.g., 107 years for the 2007 Japanese birth cohort, faces fatal inconsistencies because of implications regarding maximal lifespan. 4. The hypothesis of past and future linear life expectancy increases under critics Beyond these inconsistencies, the hypothesis of a linear increase of the record life expectancy since 1840 (Oeppen and Vaupel, 2002) and in the next decades has been criticized. Regarding past increases, Vallin and Meslé (2009) showed, after removing questionable data of New Zealand and Norway and estimating life expectancy increases not only after 1840 but from 1750, that the yearly increase of the maximum life expectancy of women was 0.5% in the 1750–1790 period, 12% in 1790–1885, 32% in 1886–1960, and 23% in 1960–2005 (i.e. ca. 3 months a year). In other words, a monotonous linear increase since 1840 does not seem to exist, and the life expectancy increase appears to be less important today than in the first half of the 20th century. Regarding future increases, Christensen et al. (2009) did not indicate whether life expectancy could stop to increase for cohorts
É. Le Bourg / Ageing Research Reviews 11 (2012) 325–328
born after 2007. Wondering whether there are limits to life expectancy is the obvious implication of their forecasts but, oddly enough, these authors did not envisage this question. If there were no limit to life expectancy, longevity would be the first biological trait with no upper limit, as it can be the case for variables in physics such as temperature. In biology, however, variables rather show an asymptotic limit, as it is observed for world records in sports for which the progression rate is not linear but exponentially decaying (Berthelot et al., 2008, see also Olshansky and Carnes, 2001 for an example on the one mile completion time). Asymptotic limits are not only a rule for sport records but also for mean height at adult age, because the rate of increase between successive birth cohorts is decreasing in younger cohorts of the European countries known to have the tallest people (e.g. Denmark or Germany) and mean height could plateau soon, if not already happened (Komlos and Lauderdale, 2007). It is also not plausible that lifespan could increase with no upper limit because, in each species, lifespan is involved in life-history strategies: there is a covariation of lifehistory strategies in mammals with, on one side of a continuum, short-lived species of small body size, early maturing with short gestation period and giving birth at short intervals to numerous offspring (e.g. most of rodents) and, on the other side, species (e.g. elephants, humans) with opposite life-history parameters (e.g. Stearns, 1983). As emphasized by Olshansky and Carnes (2005), “structural and functional constraints exist at every level of biological organization. . . within an individual, and it is their existence that imposes practical (i.e. probabilistic) limits on the lifespan of individuals and the life expectancy of populations”. Beyond the question of asymptotic limits to life expectancy, the hypothesis that life expectancy could increase in the next decades at the same pace than in the past has also been criticized by Carnes et al. (2003) who emphasized that the spectacular life expectancy increases observed during the 20th century were mainly explained by decreased childhood death rates and success against infectious diseases (extrinsic mortality): since “heart disease, cancer, stroke and diabetes (intrinsic mortality) dominate the mortality schedule today. . . there is no reason to expect that these two fundamentally different categories of death should or would adhere to the same mortality trend” (see also Vallin and Meslé, 2010). Therefore, it seems that there was not a monotonous linear life expectancy increase since 1840 and that the hypothesis of such an increase in the next decades is untenable: this hypothesis relies on computation, but not on biology. 5. What hypotheses for life expectancy after 2060? If life expectancy would not increase in the next decades in a linear way to reach more than 105 years (Christensen et al., 2009), how could it evolve? Scenarios for the French population have been published (Blanpain and Chardon, 2010): in 2060, the central scenario predicts that women’s life expectancy could be 91.1 years. What could be life expectancy beyond 2060, if we exclude the hypothesis that it could reach 105 years or more for the current youngest birth cohorts? 5.1. A longevity plateau? Life expectancy could plateau at a slightly higher value and not change thereafter, if we except yearly erratic variations. This hypothesis thus means that a life expectancy of ca. 95 years (?) would be the asymptotic limit of Homo sapiens. It also means that a slight maximal lifespan increase could be observed and that the Jeanne Calment’s record could be broken, as in the forecast of Vallin and Caselli (1997) which preserves the shape of the survival curve. If maximal lifespan would remain the same as today (ca. 115 years),
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the variability above the modal age of death would still decrease (see Cheung et al., 2009). In any case, these hypotheses admit that biomedical progress could improve health at old age, but not increase life expectancy beyond some asymptotic limit (see Holliday, 2007). The forecast of a ca. 95 life expectancy is thus not compatible with speculations that transferring results obtained on ectotherms with a very plastic longevity to homeotherms like human beings allow “to think of ageing as a disease that can be cured, or at least postponed” (Guarente and Kenyon, 2000). This hypothesis of a longevity plateau appears to be in accordance with the facts that life history variables co-vary and that biological traits have asymptotic limits (see above), i.e. with basic knowledge in biology. As lifespan co-varies with other traits such as fecundity or size, different mammalian species have different maximal lifespans: ca. 3 years in mice or 70 years in elephants, and ca. 120 years in humans. Despite progress in lab husbandry for decades, mice do not live for 20 years and no human has ever lived for 200 years. The existence of species-specific maximal lifespans in mammals is sufficient to argue that lifespan (either median or maximal) will plateau. 5.2. No longevity plateau? Life expectancy could increase regularly with no limit after 2060, but not necessary at the same pace as in previous decades. This implies that, some day, life expectancy could be higher than 105, 110, 115 years of age and so on. The main difference with the hypothesis of, e.g., Christensen et al. (2009) is then only the time scale and this hypothesis implies that maximal lifespan would also strongly increase. This very strong hypothesis means that medicine would be able to discard millions of years of evolution which made that different mammalian species have different maximal lifespans and that longevity of mammals is not so plastic as that of insects for instance (see e.g. Carey, 2002). This hypothesis would also imply that theories explaining life-history strategies (e.g. Stearns, 1983; Demetrius, 2005) are totally wrong, because a life-history trait such as longevity could increase freely and thus be disconnected from other life-history traits or constraints. In other words, accepting this hypothesis of an ever increasing human life expectancy would imply that, in human beings, longevity is not involved in trade-offs with other life-history traits, contrary to what is observed in all mammalian species. Carnes et al. (2003) have emphasized that life expectancy “is the product of an evolutionary history that established the tempo of growth, development and maturation needed to survive and reproduce” but, if human longevity could increase with no limit, one could conclude that natural selection has probably not shaped human longevity. If so, human beings would be a very special species, unless to accept that longevity can increase with no limit in all mammalian species. Indeed, the idea of an ever increasing human longevity seems to be untenable. 6. Conclusion Forecasting a not so remote asymptotic limit to maximal lifespan and life expectancy is maybe the less risky hypothesis, but only time will tell us whether this rationale is correct or not. In the meantime, colleagues trying to forecast life expectancy in the next decades should not forget to evaluate the implications for maximal lifespan and whether their hypotheses are in accordance with current thinking in evolutionary biology. Proposing a strongly increased life expectancy imposes to indicate what would be maximal lifespan and, if the expected maximal lifespan is improbable, to give up the hypothesis. For instance, among other scenarios, Vallin
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and Caselli (1997) hypothesized that life expectancy could be 150 years in 2190 with an extension of the ages at death, the last survivors dying at 270 years of age. This hypothesis thus implied that 90-year-old people living today could still live for 180 years, which is highly doubtful and is sufficient to discard it. References Berthelot, G., Thibault, V., Tafflet, M., Escolano, S., El Helou, N., Jouven, X., Hermine, O., Toussaint, J.F., 2008. The Citius End: world records progression announces the completion of a brief ultra-physiological quest. PLoS ONE 3, e1552. Blanpain, N., Chardon, O., 2010. Projections de population à l’horizon 2060. INSEE Première n◦ 1320. Carnes, B.A., Olshansky, S.J., Grahn, D., 2003. Biological evidence for limits to the duration of life. Biogerontology 4, 31–45. Carey, J.R., 2002. Longevity. The Biology and Demography of Lifespan. Princeton University Press, Princeton. Cheung, S.L.K., Robine, J.M., Paccaud, F., Marazzi, A., 2009. Dissecting the compression of mortality in Switzerland, 1876–2005. Demogr. Res. 21, 569–598. Christensen, K., Doblhammer, G., Rau, R., Vaupel, J.W., 2009. Ageing populations: the challenges ahead. Lancet 374, 1196–1208. Demetrius, L., 2005. Of mice and men. EMBO Reports 6, S39–S44. Finch, C.E., Kirkwood, T.B.L., 2000. Chance, Development, and Aging. Oxford University Press, New York. Gampe, J., 2010. Human mortality beyond age 110. In: Maier, H., Gampe, J., Jeune, B., Robine, J.M., Vaupel, J.W. (Eds.), Supercentenarians. Springer, Berlin, pp. 219–230. Guarente, L., Kenyon, C., 2000. Genetic pathways that regulate ageing in model organisms. Nature 408, 255–262. Holliday, R., 2007. Aging: the Paradox of Life. Springer, Berlin. Jeune, B., Kannisto, V., 1997. Emergence of centenarians and super-centenarians. In: Robine, J.M., Vaupel, L.W., Jeune, B., Allard, M. (Eds.), Longevity: to the Limits and Beyond. Springer, Berlin, pp. 76–89. Kirkwood, T.B.L., 1997. Is there a biological limit to the human lifespan? In: Robine, J.M., Vaupel, L.W., Jeune, B., Allard, M. (Eds.), Longevity: to the Limits and Beyond. Springer, Berlin, pp. 69–76. Kirkwood, T.B.L., Feder, M., Finch, C.E., Franceschi, C., Globerson, A., Klingenberg, C.P., LaMarco, K., Omholt, S., Westendorp, R.G.J., 2005. What accounts for the
wide variation in lifespan of genetically identical organisms reared in a constant environment? Mech. Ageing Dev. 126, 439–443. Komlos, J., Lauderdale, J.E., 2007. Underperformance in affluence: the remarkable relative decline in U.S. heights in the second half of the 20th century. Soc. Sci. Quart. 88, 283–305. Oeppen, J., Vaupel, J.W., 2002. Broken limits to life expectancy. Science 296, 1029–1031. Olshansky, S.J., Carnes, B.A., 2001. The Quest for Immortality. Norton & Co., New York. Olshansky, S.J., Carnes, B.A., 2005. Anti-aging medicine and the quest for immortality. In: Rattan, S.I.S. (Ed.), Aging Interventions and Therapies. World Scientific, Singapore, pp. 397–411. Olshansky, S.J., Carnes, B.A., Cassel, C., 1990. In search of Methuselah: estimating the upper limits to human longevity. Science 250, 634–640. Stearns, S.C., 1983. The influence of size and phylogeny on patterns of covariation among life-history traits in the mammals. OIKOS 41, 173– 187. Thatcher, R., 2010. The growth of high ages in England and Wales, 1635–2106. In: Maier, H., Gampe, J., Jeune, B., Robine, J.M., Vaupel, J.W. (Eds.), Supercentenarians. Springer, Berlin, pp. 191–201. Thatcher, A.R., Cheung, S.L.K., Horiuchi, S., Robine, J.M., 2010. The compression of deaths above the mode. Demogr. Res. 22, 505–538. Vallin, J., Caselli, G., 1997. Towards a new horizon in demographic trends: the combined effects of 150 years life expectancy and new fertility models. In: Robine, J.M., Vaupel, L.W., Jeune, B., Allard, M. (Eds.), Longevity: to the Limits and Beyond. Springer, Berlin, pp. 29–68. Vallin, J., Meslé, F., 2009. The segmented trend of highest life expectancy. Pop. Dev. Rev. 35, 159–187. Vallin, J., Meslé, F., 2010. Will life expectancy increase indefinitely by three months every year? Population and Societies n◦ 473. Vaupel, J.W., 1997a. The average French baby may live 95 or 100 years. In: Robine, J.M., Vaupel, L.W., Jeune, B., Allard, M. (Eds.), Longevity: to the Limits and Beyond. Springer, Berlin, pp. 11–27. Vaupel, J.W., 1997b. The remarkable improvements in survival at older ages. Philos. Trans. Roy. Soc. Lond. B 352, 1799–1804. Vaupel, J.W., 2010. Biodemography of human ageing. Nature 464, 536–542. Wilmoth, J.R., 2000. Demography of longevity: past, present, and future trends. Exp. Gerontol. 35, 1111–1129. Wilmoth, J.R., Deegan, L.J., Lundstrom, H., Horiuchi, S., 2000. Increase of maximum life-span in Sweden, 1861–1999. Science 289, 2366–2368.
Contents lists available at SciVerse ScienceDirect
Ageing Research Reviews journal homepage: www.elsevier.com/locate/arr
Review
Forecasting continuously increasing life expectancy: What implications? Éric Le Bourg ∗ Université Paul-Sabatier, Centre de Recherche sur la Cognition Animale, UMR CNRS 5169, 118 route de Narbonne, F-31062 Toulouse Cedex 9, France
a r t i c l e
i n f o
Article history: Received 14 December 2011 Received in revised form 16 January 2012 Accepted 19 January 2012 Available online 1 February 2012 Keywords: Human beings Median lifespan Maximal lifespan Forecasts
a b s t r a c t It has been proposed that life expectancy could linearly increase in the next decades and that median longevity of the youngest birth cohorts could reach 105 years or more. These forecasts have been criticized but it seems that their implications for future maximal lifespan (i.e. the lifespan of the last survivors) have not been considered. These implications make these forecasts untenable and it is less risky to hypothesize that life expectancy and maximal lifespan will reach an asymptotic limit in some decades from now. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Forecasting human longevity is a risky business and there is a long-lasting debate on the existence of limits to both life expectancy at birth (e.g. Olshansky et al., 1990) and maximal lifespan (e.g. Kirkwood, 1997), i.e. the lifespan of the last survivors, which is currently ca. 115 years (if we except the record-woman Jeanne Calment, who died at 122 years of age, Fig. 1). Some demographers have hypothetized that life expectancy could reach 100 years or more in the youngest birth cohorts, and these ideas have attracted much interest from mass media, as well as provoking reactions of other authors (e.g. Olshansky and Carnes, 2001; Carnes et al., 2003). These last authors have rightly relied on biological evidence to refute the idea of an always increasing lifespan. The purpose of this article is to show that, beyond these biological arguments, this hypothesis of a very long life expectancy is logically untenable because of implications regarding maximal lifespan and evolutionary biology.
observed each year among all countries, i.e. the “record female life expectancy” has increased in a linear way since 1840, the same result being also observed for males. Going a step further, they wrote that “one reasonable scenario would be that this trend will continue in coming decades. If so, record life expectancy will reach 100 in about six decades”. Christensen et al. (2009) wrote that “most babies born since 2000 in countries with long-lived residents will celebrate their 100th birthdays if the present yearly growth in life expectancy continues through the 21st century”. Table 1 in the last article thus showed that the median lifespan of French babies (both sexes together) born in 2000 could reach 102 years and that of the 2007 birth cohort 104 years, due to a 0.2 year life expectancy increase each year. As this table and the text of the article do not indicate a time limit, it can be assumed that babies of younger cohorts could even live longer, even if this hypothesis is not formally assessed (or disclaimed) by the authors. Indeed, Table 1 of the article is expecting a 107 years median lifespan for the 2007 Japanese birth cohort and thus, due to the longevity gap between genders, probably a still higher lifespan for Japanese women.
2. The forecasts for very high life expectancies. . . In 1997, Vaupel (1997a) stressed that his “best guess. . . is that half of the babies born in France this year (1996) will reach age 95 and that most of the girls will reach age 100”. A few years later, Oeppen and Vaupel (2002) stressed that previous attempts to predict life expectancy had all been surpassed by observation. They displayed a graph showing that the highest women life expectancy
∗ Fax: +33 5 61 55 61 54. E-mail address: [email protected] 1568-1637/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.arr.2012.01.002
3. .. . . and their implications for maximal lifespan What is the hypothesis of previously cited demographers regarding future maximal lifespan? Obviously, if life expectancy at birth would increase beyond 105 years, it would be very close to the current maximal lifespan but, oddly enough, these authors did not consider the evolution of maximal lifespan in their articles. However, in the box 1 of his article, Vaupel (2010) stressed that “the process of senescence is being delayed rather than being decelerated”. Consistent with this hypothesis, Figure 2 in Vaupel (2010) showed that mortality rates of most longevous sub-cohorts do not
326
É. Le Bourg / Ageing Research Reviews 11 (2012) 325–328
that maximal lifespan would not increase if median lifespan would reach, e.g. 107 years of age. To go a step further, if median lifespan would increase up to, say, 110 or 115 years of age, 50% of the birth cohort would die in a few months if maximal lifespan would not increase. Thus, the hypothesis of no maximal lifespan increase if life expectancy would reach 105 and beyond is a logical dead-end.
Maximal recorded longevity (years)
125
120
3.2. Second hypothesis: maximal lifespan increase 115
110
105 1950
1960
1970
1980
1990
2000
2010
Year of death Fig. 1. Maximal recorded human longevity from 1955 to 2010. Data are extracted from table CCCC on the Gerontology Research Group website (www.grg.org) and dubious cases of the table are not included in the figure. Data on maximal lifespan are also available from the International Database on Longevity (www.supercentenarians.org). The first confirmed super-centenarian (i.e. a person living longer than 110 years) died in 1932 at 111 years of age and probably no other super-centenarian existed before this time (Vaupel, 1997b; Jeune and Kannisto, 1997; Wilmoth et al., 2000). The highest maximal lifespan ever observed is that of Jeanne Calment, who died at 122 years of age in 1997.
differ from other cohorts by their slope and, as these longevous subcohorts age at the same rate than less longevous ones, that their mortality rates can be very low at oldest ages. Therefore, it can be probably deduced from the Vaupel’s (2010) hypothesis of the “delay not deceleration” of the aging process that maximal lifespan is expected to increase when median lifespan increases. Nevertheless, let us examine the two hypotheses regarding maximal lifespan changes: no increase or increase. 3.1. First hypothesis: no maximal lifespan increase In this hypothesis, maximal lifespan would thus remain around ca. 115 years, i.e. the maximal lifespan observed for ca. one century (Fig. 1). It follows, since median lifespan would reach e.g. 107 years for the 2007 Japanese birth cohort (Christensen et al., 2009), that 50% of this cohort would die between 107 and ca. 115 years of age. Therefore, there would be a strong compression of mortality in the next decades: as the modal age of death would increase more quickly than maximal lifespan, the variability above this modal age would decrease. Such a compression of mortality has been observed in developed countries during the last century (e.g. Wilmoth, 2000; Thatcher et al., 2010), but the variability above the modal age of death seems to have stopped to decrease in recent years in some countries (Switzerland, Japan, France or Italy, see Cheung et al., 2009). Is it biologically plausible that mortality rates would be very low up to, say, 107 years of age and, after that, would suddenly increase up to reach 50% each year, i.e. the death rates currently observed above 110 years of age (Gampe, 2010; Thatcher, 2010)? This hypothesis seems untenable because the variability of lifespan would be very low, 50% of the cohort dying in a very few years, while it is known that there is a high variability of lifespans even in inbred strains and in monozygotic human twins (Finch and Kirkwood, 2000; Kirkwood et al., 2005). It is thus difficult to accept that genetically heterogeneous people with different living conditions could die in a very narrow time span. It seems thus impossible
It thus seems that maximal lifespan should increase if median lifespan would reach, e.g. 107 years of age. The hypothesis of a maximal lifespan increase by a right-shifting of the survival curve, i.e. by maintaining the rectangularization of the survival curve, has been studied by Vallin and Caselli (1997): if life expectancy would reach 105 years of age maximal lifespan would be around 140 years (145 years if life expectancy were 110). In other words, maximal lifespan should increase by ca. 25 years in the next decades, if we consider that the most longevous supercentenarians die at ca. 115 years of age, or by ca. 20 years, if we consider that the current record of 122 years is more representative of the actual maximal lifespan. In any case, Fig. 1 shows that maximal lifespan is rather steady for decades (if we except three exceptional cases of 119 and 122 years of age), despite huge increases of life expectancy during this period, and it is difficult to admit that maximal lifespan could suddenly increase to such a large extent. 3.3. Conclusions: maximal lifespan can neither increase nor remain steady if life expectancy is above 105 It seems difficult to support the idea that maximal lifespan would not change if life expectancy would be well above 100. Anyway, predicting that maximal lifespan would increase implies to explain how reaching so high maximal values. Therefore, we are let with the strong hypothesis that median lifespan could be well above 100 for the youngest birth cohorts (and maybe more for those not still born), but with the puzzling conclusion that maximal lifespan could not remain the same as today, nor increase unless to accept that it could be much higher than the maximal lifespans observed for decades. Indeed, when hypotheses predict substantial changes (and predicting that maximal lifespan could reach much higher values than today is a substantial change), “the burden of the proof lies with those who predict sharp deviations from past trends”, as stressed by Wilmoth (2000). Therefore, it seems that the hypothesis of a continuous linear increase of life expectancy in the next decades up to reach, e.g., 107 years for the 2007 Japanese birth cohort, faces fatal inconsistencies because of implications regarding maximal lifespan. 4. The hypothesis of past and future linear life expectancy increases under critics Beyond these inconsistencies, the hypothesis of a linear increase of the record life expectancy since 1840 (Oeppen and Vaupel, 2002) and in the next decades has been criticized. Regarding past increases, Vallin and Meslé (2009) showed, after removing questionable data of New Zealand and Norway and estimating life expectancy increases not only after 1840 but from 1750, that the yearly increase of the maximum life expectancy of women was 0.5% in the 1750–1790 period, 12% in 1790–1885, 32% in 1886–1960, and 23% in 1960–2005 (i.e. ca. 3 months a year). In other words, a monotonous linear increase since 1840 does not seem to exist, and the life expectancy increase appears to be less important today than in the first half of the 20th century. Regarding future increases, Christensen et al. (2009) did not indicate whether life expectancy could stop to increase for cohorts
É. Le Bourg / Ageing Research Reviews 11 (2012) 325–328
born after 2007. Wondering whether there are limits to life expectancy is the obvious implication of their forecasts but, oddly enough, these authors did not envisage this question. If there were no limit to life expectancy, longevity would be the first biological trait with no upper limit, as it can be the case for variables in physics such as temperature. In biology, however, variables rather show an asymptotic limit, as it is observed for world records in sports for which the progression rate is not linear but exponentially decaying (Berthelot et al., 2008, see also Olshansky and Carnes, 2001 for an example on the one mile completion time). Asymptotic limits are not only a rule for sport records but also for mean height at adult age, because the rate of increase between successive birth cohorts is decreasing in younger cohorts of the European countries known to have the tallest people (e.g. Denmark or Germany) and mean height could plateau soon, if not already happened (Komlos and Lauderdale, 2007). It is also not plausible that lifespan could increase with no upper limit because, in each species, lifespan is involved in life-history strategies: there is a covariation of lifehistory strategies in mammals with, on one side of a continuum, short-lived species of small body size, early maturing with short gestation period and giving birth at short intervals to numerous offspring (e.g. most of rodents) and, on the other side, species (e.g. elephants, humans) with opposite life-history parameters (e.g. Stearns, 1983). As emphasized by Olshansky and Carnes (2005), “structural and functional constraints exist at every level of biological organization. . . within an individual, and it is their existence that imposes practical (i.e. probabilistic) limits on the lifespan of individuals and the life expectancy of populations”. Beyond the question of asymptotic limits to life expectancy, the hypothesis that life expectancy could increase in the next decades at the same pace than in the past has also been criticized by Carnes et al. (2003) who emphasized that the spectacular life expectancy increases observed during the 20th century were mainly explained by decreased childhood death rates and success against infectious diseases (extrinsic mortality): since “heart disease, cancer, stroke and diabetes (intrinsic mortality) dominate the mortality schedule today. . . there is no reason to expect that these two fundamentally different categories of death should or would adhere to the same mortality trend” (see also Vallin and Meslé, 2010). Therefore, it seems that there was not a monotonous linear life expectancy increase since 1840 and that the hypothesis of such an increase in the next decades is untenable: this hypothesis relies on computation, but not on biology. 5. What hypotheses for life expectancy after 2060? If life expectancy would not increase in the next decades in a linear way to reach more than 105 years (Christensen et al., 2009), how could it evolve? Scenarios for the French population have been published (Blanpain and Chardon, 2010): in 2060, the central scenario predicts that women’s life expectancy could be 91.1 years. What could be life expectancy beyond 2060, if we exclude the hypothesis that it could reach 105 years or more for the current youngest birth cohorts? 5.1. A longevity plateau? Life expectancy could plateau at a slightly higher value and not change thereafter, if we except yearly erratic variations. This hypothesis thus means that a life expectancy of ca. 95 years (?) would be the asymptotic limit of Homo sapiens. It also means that a slight maximal lifespan increase could be observed and that the Jeanne Calment’s record could be broken, as in the forecast of Vallin and Caselli (1997) which preserves the shape of the survival curve. If maximal lifespan would remain the same as today (ca. 115 years),
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the variability above the modal age of death would still decrease (see Cheung et al., 2009). In any case, these hypotheses admit that biomedical progress could improve health at old age, but not increase life expectancy beyond some asymptotic limit (see Holliday, 2007). The forecast of a ca. 95 life expectancy is thus not compatible with speculations that transferring results obtained on ectotherms with a very plastic longevity to homeotherms like human beings allow “to think of ageing as a disease that can be cured, or at least postponed” (Guarente and Kenyon, 2000). This hypothesis of a longevity plateau appears to be in accordance with the facts that life history variables co-vary and that biological traits have asymptotic limits (see above), i.e. with basic knowledge in biology. As lifespan co-varies with other traits such as fecundity or size, different mammalian species have different maximal lifespans: ca. 3 years in mice or 70 years in elephants, and ca. 120 years in humans. Despite progress in lab husbandry for decades, mice do not live for 20 years and no human has ever lived for 200 years. The existence of species-specific maximal lifespans in mammals is sufficient to argue that lifespan (either median or maximal) will plateau. 5.2. No longevity plateau? Life expectancy could increase regularly with no limit after 2060, but not necessary at the same pace as in previous decades. This implies that, some day, life expectancy could be higher than 105, 110, 115 years of age and so on. The main difference with the hypothesis of, e.g., Christensen et al. (2009) is then only the time scale and this hypothesis implies that maximal lifespan would also strongly increase. This very strong hypothesis means that medicine would be able to discard millions of years of evolution which made that different mammalian species have different maximal lifespans and that longevity of mammals is not so plastic as that of insects for instance (see e.g. Carey, 2002). This hypothesis would also imply that theories explaining life-history strategies (e.g. Stearns, 1983; Demetrius, 2005) are totally wrong, because a life-history trait such as longevity could increase freely and thus be disconnected from other life-history traits or constraints. In other words, accepting this hypothesis of an ever increasing human life expectancy would imply that, in human beings, longevity is not involved in trade-offs with other life-history traits, contrary to what is observed in all mammalian species. Carnes et al. (2003) have emphasized that life expectancy “is the product of an evolutionary history that established the tempo of growth, development and maturation needed to survive and reproduce” but, if human longevity could increase with no limit, one could conclude that natural selection has probably not shaped human longevity. If so, human beings would be a very special species, unless to accept that longevity can increase with no limit in all mammalian species. Indeed, the idea of an ever increasing human longevity seems to be untenable. 6. Conclusion Forecasting a not so remote asymptotic limit to maximal lifespan and life expectancy is maybe the less risky hypothesis, but only time will tell us whether this rationale is correct or not. In the meantime, colleagues trying to forecast life expectancy in the next decades should not forget to evaluate the implications for maximal lifespan and whether their hypotheses are in accordance with current thinking in evolutionary biology. Proposing a strongly increased life expectancy imposes to indicate what would be maximal lifespan and, if the expected maximal lifespan is improbable, to give up the hypothesis. For instance, among other scenarios, Vallin
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