By Professor Sarah Harper

The past decade has seen a growing public awareness of the ageing of the population among most industrialized countries. What is less well understood is that this is not just a North American and European phenomenon, but is now occurring in Asia and Latin America. In addition, the ageing of populations arises not so much from increased longevity, but through falling fertility. As a result the 21st century is likely to see not a population explosion, but a cessation of population growth altogether.

Thirty years ago the overwhelming demographic question was how we could prevent world population from growing to more than 20 billion. Now the defining demographic characteristics of the 21st century are likely to be declining births and stabilization in size.

World population is predicted to increase from the current 7 billion to around 10 billion by the middle of the century, the growth flattening and remaining thus until the end of the century.

The global distribution of people will also change, with an overall increase in those living in Asia and Africa, and a fall in European and North American populations. The less and least developed region countries will account for 97 per cent of the growth to 2050. Asia will comprise 55 per cent of the world population by 2050 at 5 billion, Africa is projected to double in size by 2050 from 1 billion to 2 billion, while Europe will decline from 738 million to 719 million.

The age composition of the population will also alter as median ages rise, and there is a proportionate shift from younger to older people across the globe. Around a quarter of the world's population will be over 60 by 2050. Two thirds of these people will live in Asia, which by then will have more people aged over 60 than under 15.

These changes in the demography of human populations have arisen due to the demographic transition. This started in Europe sometime after 1750, in Asia and Latin America during the 20th century, and now there are indications that Africa will transition during this century. Why the demographic transition occurred when it did, where it did and how it did is strongly debated. However, as humans develop economically, mortality falls, populations start to grow and then fertility falls.

The key to the demographic transition is changes in the number of children born to women of childbearing age. The actual drivers of declines in childbearing have long been debated but broadly fall into three positions. One theory is that childbearing falls in response to a reduction in infant mortality. In other words, an increase in child survival rates reduces the number of births required to achieve the desired number of surviving children.

A second position is that the introduction of modern family planning methods has allowed women to choose the number of births they have. The third broad hypothesis is that fertility fall is driven by education. There is a strong association between those countries with a high level of educated women, and those with below replacement fertility levels.

Similarly, those countries with low rates of female education have high rates of childbearing. Educating girls, in particular, encourages later marriage and gives them access to the labour market, which reduces the number of births, but also and crucially, changes the mind set of the women and their communities and enables them to recognize the range of alternative choices they can make.

Two thirds of the world's countries now have childbearing rates that are at or below replacement level -- crudely defined as 2.1. These are diverse, including Hong Kong, the lowest at 0.99, Poland, Germany, Barbados, Thailand, Vietnam, Mauritius, Iran, Chile, Tunisia, and the United States. Indeed some demographers now argue that low fertility countries are in the midst of a second demographic transition that is keeping fertility well below replacement. This may be due to technological advances and changes in the labour market that have altered the costs and rewards of marriage and child rearing. Others suggest that it may be ideational changes that have accompanied our increased affluence leading to a focus on individual autonomy and self-realization. In particular the evolutionary link between the sex drive and procreation has clearly been broken through the introduction of modern contraception, and now reproduction is merely a function of individual preferences and culturally determined norms.

Indeed some countries may be in a so-called low fertility trap which arises both through demographic factors, the fact that fewer potential mothers in the future will result in fewer births, and sociological ones in that ideal family size for the younger generations is declining as a consequence of the lower childbearing they see in previous generations.

Here it is argued that countries with very low childbearing rates of below 1.5 for more than one generation become adapted to childless or one-child families, and it is then very difficult to raise fertility again. Employment patterns change, childcare provision and schools diminish and so on. There is limited empirical evidence for this theory; however we do have anecdotal evidence emerging from China. Here the one-child policy has been in place for 30 years, and we thus have a large number of one-child children who are now of childbearing age. Despite the fact that they are allowed two children, it seems that many are choosing to have only one child themselves because this is their own experience. They grew up in a society of single children.

Such low childbearing will impact upon future population structures. For example, by the end of the century the percentage of children aged between 5 and 14 in most countries will have fallen to 15 per cent or below, and those of working age, currently defined by the UN as between 15 and 64, will have fallen to just over half. The caveat here is that of course 15 to 64 is an unrealistic proxy for working age even today, and it is highly improbable that it will define working life by the middle of the century. But this is one of the key messages: unless policies support a change in behaviour, this will be the scenario. The reason why this is seen as important is that generally productive capacity varies across the life course, flowing from a period of youth dependency, through high productive potential in adulthood, returning to a decrease in productivity in old age.

When we are young adults we produce, consume and save, and when we are older we reduce our production and consumption and begin to draw down on our investments.

However, while population ageing is primarily driven by a decrease in child bearing, mortality rates have also been falling and this too has significant impacts.

Some argue that humans have been designed to live long enough to reproduce and ensure the survival of their offspring. In an ageing population, however, most of us live well beyond this 'essential life span'. The 21st century is likely to see life extension become a common experience. Indeed life expectancy in the developed world is increasing by five hours a day, two years a decade. A 70-year-old European, for example, has the same probability of dying today as a 57-year-old 50 years ago.

A key question, however, is whether these falls in mortality are accompanied by falls in morbidity, which is disease and disability. There is currently evidence that through healthy living and disease prevention, the onset of disability is being pushed back into our 80s.

Will these gains in healthy years continue as we increasingly turn to science and technology to extend our years? Or will the modern drivers of longevity not only increase our life expectancy but also enable us to live for longer and longer with disease, disabilities and frailties. Some argue that we are potentially heading for a future in which we are able through science to keep people with chronic conditions alive for longer and longer, and if this is the case how can we refigure our families, our communities and our societies to support these new extended lives?

There is also the question of radical longevity. This is the extension of natural life spans to 120, 150, 200 . . . raising some key questions. For example, will life expectancy increase in line with life extension? Will we all enjoy the benefits of longevity or will it be for a few?

The drivers of life extension include healthy living, disease prevention and cure, and regenerative medicine. It appears that already healthy living and disease prevention and cure may push many lives in the developed world to the century. For example, the number of centenarians in Britain is likely to increase from around 12,000 currently to near half a million by middle of century and approaching one million by the end. Eight million people alive in Britain are likely to reach a century and 127 million in Europe. According to some demographers, the life expectancy of babies born today is probably around 103.

Another key question is whether increases in life expectancy will be accompanied by increases in life extension or whether we are seeing a compression of longevity after 100. In other words will the predicted increases in centenarians over the coming century be accompanied by increases in super-centenarians living to over 110? Evidence suggests mortality rates at extreme old age in many developed countries are declining and show no signs of slowing. In those countries such as Japan where there are sufficient numbers of very old people, we find evidence of a 'shifting mortality scenario'. As we see an increase in centenarians, so we should expect to see an increase in super-centenarians.

This then is the demography of the 21st century -- low fertility and low mortality rates for most countries, leading to an ageing of the world's population. How can modern societies successfully adjust to population ageing?

A key challenge will be the provision of health and social care. In particular the amount of ill health and disability is likely to increase, the type of ill health to change from acute to chronic conditions and the effect of falling numbers of younger people will reduce those able to provide care.

The provision of income support presents another set of challenges. Some are now arguing that we need to reconsider the traditional generational contract whereby adults provide for children, and in return when those young dependents become adults they provide for older dependents. This is maintained in most societies, both within families and beyond, with working adults providing education, health care, and income support for young and old dependents via taxation.

The question for an ageing population is whether current generations, who have been successful in reducing both their fertility and mortality, should pass on the cost of such success to future generations via this traditional intergenerational contract, or whether they should bear the cost of their success via an adapted intergenerational contract. This would require older people to take on the costs of their longer lives by providing for themselves in old age, through working longer for example.

Perhaps the key public policy question is how current policy goals will influence these required societal adjustments. Europe, which has had more than a hundred years to prepare for its ageing population, is still struggling with this question. Of importance is the rapidly changing demography of Asia and Latin America, regions moving from being predominantly young to being predominantly old over the next 25 years. This is where the greatest challenge of global ageing is likely to lie.

Professor Sarah Harper is Director of the Oxford Institute of Population Ageing at the University of Oxford


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