Many people believe that overpopulation is the greatest threat to the world's security and prosperity. It was probably Malthus who first pointed out that population growth is exponential, while agricultural growth is arithmetic. He reasoned that every population must inevitably outgrow its food source. Political instability and border insecurity naturally follow, when growing populations seek to nervously protect their shrinking resource base. Critics of Malthus pointed out that human populations, unlike bacteria, do not grow exponentially; fertility rates vary geographically and over time. New economic models, including those that factor labour as capital, valued large populations as assets rather than detriments, allowing industrializing nations to convert population to production and therefore to wealth, allowing them to purchase needed resources.

(Photo: Daniel V. Fung via Getty Images) The predicted Malthusian collapse did not occur, due largely to improvements in agricultural sciences and something called the Demographic Transition. It's this latter thing that gives many demographers hope that the world's population growth might be slowing, soon to be plateauing, and eventually reversing. The Demographic Transition was first proposed by Warren Thompson in the 1920s and is well described on BBC's educational site. From observing changes in Europe over the centuries, Thompson and his intellectual heirs suggested that societies evolve through five stages of demographic development. In Stage 1, we live pre-industrial lives, dependent on the land and at the mercy of the elements. Diseases are plenty, lifespan is short. Infant mortality rates are very high, such that we don't even name our children until we are sure they will live beyond infancy. The cost of a child is simply the price of feeding that child, whereas its value as a labourer is high. Both death rates and birth rates are high, and the population is neither growing nor shrinking. So many countries have entered Stage 3 that the human race is now a majority urban species. In Stage 2, with the arrival of public health and hygiene measures, infectious diseases recede, and mortality declines, particularly among children. But the cost of children is still low, while their utility is high. Thus, reproduction behaviours remain unchanged. But overall population size increases dramatically. Many so-called developing countries would have been in Stage 2 in the late 19th and early 20th centuries, accounting for their explosive growth. In Stage 3, social factors intervene to lower the birth rate. When infants survive and women are socially empowered, fertility rates drop. As economies shift from agrarian to industrial bases, more people move to cities. In fact, so many countries have entered Stage 3 that the human race is now a majority urban species. The widespread introduction of contraception methods, and the use of new wealth to invest in public education (especially for women), leads to smaller families. The cost of children increases with housing and educational demands, while their utility diminishes, as they are no longer needed as labourers. Thus, in Stage 3 population size is still increasing, but more slowly. Western Europe began emerging from Stage 3 in the 19th century. India and China, with their expanding middle class, are in the latter half of Stage 3 or beyond.

(Photo: Peterv via Getty Images) In Stage 4, we transition to a services and information economy, with few economic incentives for reproduction. Birth and death rates are balanced, and there is minimal population growth. Canada is likely a Stage 4 nation. Rapidly modernizing Stage 3 nations may be progressing to Stage 4 quickly. China is approaching a sort of demographic cliff, since a majority of its people are over the age of 30 and are having fewer children than their recent ancestors. Some recognize a fifth stage in which birth rate drops below death rate, and the population shrinks. Japan is a Stage 5 country. Subsequent generations are too small to provide an adequate tax base for maintaining social programs. Immigration policies must be re-thought, and governments begin to experiment with strategies for encouraging larger families. The Demographic Transition is a theoretical construct with some vulnerabilities. It is unclear whether deep religiosity can compel modernized generations to reproduce at rates against their economic self-interest, leading to what some call a "demographic trap." Total world population should plateau at about 13 billion by 2100, and actually decline thereafter. Also, it is possible that globalization is a disincentive for modernization, compelling some populations to remain agrarian or manufacturing-based, never to progress to an information-based economy. Climate change, antibiotic resistance, and the unpredictability of international affairs can confound a nation's linear progression through the stages. However, given that the basic tenets of the Demographic Transition are being observed in real time in such living social laboratories as India, China, and parts of Africa, it is reasonable that the world is in the process of transitioning. With investments in public health, reproductive rights, and wealth-building programs, we have seen dramatic recent reductions in the global fertility rate. Most of humanity is transitioning to Stage 4, which means that while total population size is still growing, growth is slowing down. Human population growth is expected to drop to one per cent by 2020, though we are still on schedule to reach the 9 billion mark by 2050. Total world population should plateau at about 13 billion by 2100, and actually decline thereafter.