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Online ISSN : 2424-1903
Print ISSN : 0388-0036
人口転換論を再考する
――とくに死亡率低下局面をめぐって
斎藤 修
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ジャーナル フリー

2018 年 73 巻 1 号 p. 1-

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Mortality decline, the first phase of the demographic transition, has long been considered a transition triggered by the industrial revolution. This paper is an attempt to question this traditional view, putting forward a set of arguments, first, that the mortality transition was a longer process than conventionally thought, starting a century earlier than the onset of modern economic growth, and second, that a substantial increase in the level of life expectancy at birth, a summary measure of age-specific mortality rates, was indeed a prerequisite of modern economic growth and, hence, of fertility transition, the second phase of the demographic transition.
 The article begins with a brief overview of the historiography in demography, epidemiology and medical history. Much effort has been made to account for the historical process of mortality decline, and from the past debates there are a couple of lessons learned. One is that while medical progress can no longer explain the early decline in mortality, public health measures are likely to have played an important part. The other is that there is not much evidence to support the claim that economic development led to reductions in death rates. The demographer Samuel Preston demonstrated, by assembling country data on both life expectancy at birth and national income per capita for the 1930s and the 1960s, that the curve representing the relationship between the two factors (called the Preston curve) shifted significantly upwards between the two time-periods. According to his analysis of inter-temporal shifts, income growth per se explained only 16 per cent of the rise in longevity while a bundle of other factors accounted for 84 per cent; and the only variable that had a statistically significant correlation with the gain in life expectancy was the initial level of income – though, contrary to the expectation, its sign was negative. There are also suggestions by historians that in earlier stages where epidemics were more frequent nutrition is unlikely to have been a decisive factor to bring the level of mortality down.
 According to a recent synthesis in evolutionary biology of pathogens, there is a trade-off between their lethality and transmission potential. Diseases with high lethality, such as plague and smallpox, do not usually depend on their human host for transmission, whereas pathogens that are transmitted person to person but cannot persist outside the host are under strong selective pressure against high host lethality. Based on this conceptual framework as well as stylised facts from English historical demography, Romola Davenport of the Cambridge Group for the History of Population and Social Structure argues that infectious diseases reduced or eliminated in periods before 1870 are those of the first type, while childhood infections and other diseases that lingered on into the first half of the twentieth century are of the second type. Indeed, the Cambridge Group’s path-breaking work on the historical demography of England, c.1540-1870, reveals that from the late seventeenth century onwards adult, child and neonatal mortality rates started to decline but the post-neonatal mortality rate did not. Over the long run, in other words, deaths from pestilence and famine were reduced, thanks to early modern economic growth or the implementation of rudimentary measures in public health by government authorities, or both, while exposure to infection was on the rise, suggesting that the long-term decline of mortality started well before the industrial revolution, but the process became prolonged as industrialisation and urbanisation progressed.
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