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Population ageing is a phenomenon that occurs when the median age of a country or region rises due to rising life expectancy and/or declining fertility rates. There has been, initially in the more economically developed countries (MEDC) but also more recently in less economically developed countries (LEDC), an increase in life expectancy which causes the ageing of populations. This is the case for every country in the world except the 18 countries designated as "demographic outliers" by the UN. For the entirety of recorded human history, the world has never seen as aged a population as currently exists globally.〔(World Population Ageing: 1950-2050 ), United Nations Population Division.〕 The UN predicts the rate of population ageing in the 21st century will exceed that of the previous century.〔 Countries vary significantly in terms of the degree, and the pace, of these changes, and the UN expects populations that began ageing later to have less time to adapt to the many implications of these changes.〔 ==Overview== Population ageing is a shift in the distribution of a country's population towards older ages. This is usually reflected in an increase in the population's mean and median ages, a decline in the proportion of the population composed of children, and a rise in the proportion of the population that is elderly. Population ageing is widespread across the world. It is most advanced in the most highly developed countries, but it is growing faster in less developed regions, which means that older persons will be increasingly concentrated in the less developed regions of the world.〔(【引用サイトリンク】title=World Population Ageing 2013 )〕 The Oxford Institute of Population Ageing, however, concluded that population ageing has slowed considerably in Europe and will have the greatest future impact in Asia, especially as Asia is in stage five of the demographic transition model. Among the countries currently classified by the United Nations as more developed (with a total population of 1.2 billion in 2005), the overall median age rose from 28 in 1950 to 40 in 2010, and is forecast to rise to 44 by 2050. The corresponding figures for the world as a whole are 24 in 1950, 29 in 2010, and 36 in 2050. For the less developed regions, the median age will go from 26 years in 2010 to 35 years in 2050.〔(【引用サイトリンク】title=World Ageing Population 2013 )〕 Population ageing arises from two (possibly related) demographic effects: increasing longevity and declining fertility. An increase in longevity raises the average age of the population by increasing the numbers of surviving older people. A decline in fertility reduces the number of babies, and as the effect continues, the numbers of younger people in general also reduce. Of these two forces, it is declining fertility that is the largest contributor to population ageing in the world today.〔Weil, David N., "The Economics of Population Aging" in Mark R. Rosenzweig and Oded Stark, eds., Handbook of Population and Family Economics, New York: Elsevier, 1997, 967-1014.〕 More specifically, it is the large decline in the overall fertility rate over the last half century that is primarily responsible for the population ageing in the world’s most developed countries. Because many developing countries are going through faster fertility transitions, they will experience even faster population ageing than the currently developed countries in the future. The rate at which the population ages is likely to increase over the next three decades; however, few countries know whether their older populations are living the extra years of life in good or poor health. A "compression of morbidity" would imply reduced disability in old age,〔 〕 whereas an expansion would see an increase in poor health with increased longevity. Another option has been posed for a situation of "dynamic equilibrium".〔(【引用サイトリンク】title=Manton, 1982 )〕 This is crucial information for governments if the limits of lifespan continue to increase indefinitely, as some researchers believe it will.〔 〕 The World Health Organization's suite of household health studies is working to provide the needed health and well-being evidence, including, for example the World Health Survey, and the Study on Global Ageing and Adult Health (SAGE). These surveys cover 308,000 respondents aged 18+ years and 81,000 aged 50+ years from 70 countries. The Global Ageing Survey, exploring attitudes, expectations and behaviours towards later life and retirement, directed by George Leeson, and covering 44,000 people aged 40–80 in 24 countries from across the globe has revealed that many people are now fully aware of the ageing of the world's population and the implications which this will have for their lives and the lives of their children and grandchildren. Canada has the highest per capita immigration rate in the world, partly to counter population ageing. The C. D. Howe Institute, a conservative think tank, has suggested that immigration cannot be used as a viable mean for countering population ageing. This conclusion is also seen in the work of other scholars. Demographers Peter McDonald and Rebecca Kippen comment, "()s fertility sinks further below replacement level, increasingly higher levels of annual net migration will be required to maintain a target of even zero population growth". 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Population ageing」の詳細全文を読む スポンサード リンク
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