In order to anticipate the future, we have to understand the present and the past. To know where we are heading, we must know where we are and how we got here.

When demographers try to make sense of the complexities of the world around us, they use one of social science’s great generalizing models: the demographic transition. In association with many other aspects of modernization, every population in the world has experienced or is still undergoing a set of inter-connected changes that is termed the demographic transition. As Paul Demeny (1972) has succinctly put it: “In traditional societies, fertility and mortality are high. In modern societies, fertility and mortality are low. In between there is the demographic transition.” As a description of long-run trends, the demographic transition can be seen to be a universally applicable generalization. At some point in the past, every population had high fertility (mostly between four and six children per woman) and high mortality (life expectancy varied between 20 and 40 years). With the spread of modern medicine and public health, mortality has improved; as family planning and contraceptive use became the norm, fertility has fallen. Usually mortality fell first, with a delay before fertility decline. This difference in timing leads to substantial population growth before the two processes come back into balance. The process of transition began in the later 18th and 19th centuries in Europe and the neo-Europes overseas; it became a global phenomenon after World War II. Today, more than half the world’s people live in places where fertility is at or below the level needed for long-run inter-generational replacement (about 2.1 children per woman). Similarly, global life expectancy is approaching 70 years. Taken together, these changes amount to the most significant demographic transformation in human history.

As a consequence of the demographic transition, the 20th century was a century of unprecedented population growth – the global population grew almost fourfold, from 1.6 to 6.1 billion. However, while significant future growth is certain, the end of the demographic transition is now in sight. As an article by POP researchers in Nature in 2001 showed, it is likely that human population growth will come to an end over the course of the 21st century (Lutz et al., 2001). IIASA’s projections envisage a maximum population of around 9 billion being reached around the middle of this century. In contrast with the growth of the 20th century, the 21st will be a century of population aging. All populations that have long life expectancy and low long-run rates of population growth will experience aging. This will soon apply to all parts of the globe. Assessing the implications of this ineluctable transformation of the human age structure is a fundamental task for population science. How can we create societies that are economically and environmentally sustainable in the context of substantial aging? In carrying out this task it will be essential to consider not just quantity but also quality, i.e., not only how many people there will be, but how well endowed they are with human capital. Moreover, where the population will live will also play a key role in determining their income and lifestyles, and hence their impact on the environment. In these regards, decomposing future population according to urban-rural residence and level of educational attainment is a crucial advance on current forecasting practice.

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Last edited: 05 November 2012


Wolfgang Lutz

Program Director

World Population

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International Institute for Applied Systems Analysis (IIASA)
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