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Inaugural lectures

Chair in Sustainable Development - Environment, Energy and Society

Academic Year 2008-2009
Henri Leridon
p. 21
Cet article est une traduction de :
Chaire Développement durable – environnement, énergie et société [fr]

Notes de la rédaction

Henri Leridon gave his inaugural lecture on 5 March 2009. His course, entitled “Demography, the end of the transition?” began on 18 March 2009

Texte intégral

Extracts from the inaugural lecture:

1“Exactly 40 years ago, we experienced an event that was both unique and decisive in the history of mankind […]: the world population growth rate reached an all-time high – 2% per year – before falling back again quite rapidly.

2“[…] How can we be sure that a rate of 2% had never been achieved in the past? Because we can apply the laws of demography, most of which were established between 1920 and 1960. Let me begin with the simplest law. In a “closed” population, i.e. a population with no inward or outward migration (as is the case for the world population!) the growth rate is equal to the difference between birth and death rates. The question is therefore to determine these rates for human populations through history. To do so, we need either to have reliable statistics on these populations, or to make estimates based, among other things, on biological factors. We can also combine the two.

3“[…] Another reason for believing that the recent demographic trend is historically unprecedented is that we now understand the mechanisms involved. In practically all societies, mortality has declined, but without an immediate corresponding decrease in birth rates. It is this process of demographic transition which will form the main thread of my lectures. In short, world population growth in the second half of the twentieth century – from 2.5 to 6 billion inhabitants – is a consequence of progress and economic development in today’s most industrialized countries, which have “exported” a part of their success in combating mortality to the less developed world. Because the fertility decline in developing countries has sometimes been much slower (it has barely begun in some parts of Africa), the time lag between changes in mortality and fertility led to a period of rapid population growth that peaked in the 1960s.

4“[…] World population trends over the last 100 years have thus been exceptional. If we reason in absolute terms, growth is following a potentially alarming pattern. Planet Earth counted an additional 75 million inhabitants each year in the years 1970-90, and this figure is only now starting to decline. For Africa, the continent which today posts the highest growth rates, the population will increase from 800 million to 2 billion in just 50 years, after doubling in 25 years. It was such figures that sparked the concerns first voiced in the late 1950s and in the following decade. The issue took a new turn with the publication in 1968 of The Population Bomb by the entomologist Paul Ehrlich and his wife Anne, and triggered global debate in 1972 with The Limits to Growth by Donella and Dennis Meadows. The central theme of these two works, and the focus of their concern, was the rapid and ultimately unsustainable pace of population growth.

5“[…] But times have changed, and the debate must now shift away from the issue of “overpopulation”. To start with, the 6.6 billion inhabitants of the Earth are here to stay. It is more urgent to feed them and offer them decent living conditions than to argue about numbers. Second, the fertility decline is already well advanced: the mean number of children per woman has fallen from 5 to 2.7. In other words, we are already 80% along the road towards the “replacement level” of 2.2, which may be reached by 2030. In fact, in many industrialized countries, there is a real fear of demographic decline, and debate about the risks of excessive population growth has become irrelevant.

6“[…] But whether we accept an extensive or restrictive version of sustainable development, there is no doubt that our societies will need to adapt to new constraints and adjust their frame of reference. We will have to change our eating habits, our modes of consumption (of energy especially), drastically reduce pollution levels, and so on […]. And we must ensure that humankind remains central to the overall debate. We too often tend to forget that the economy exists to serve the needs of people and not the other way round. Development must be sustainable to enable future generations to live decently on our planet, not simply to restore the Earth to some indefinable and utopian ‘natural’ state.”

7The Chair is supported by Total.

8The inaugural lecture is available from Editions Fayard. The video is available in French and English on the College de France website.

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Référence papier

Henri Leridon, « Chair in Sustainable Development - Environment, Energy and Society »La lettre du Collège de France, 4 | 2009, 21.

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Henri Leridon, « Chair in Sustainable Development - Environment, Energy and Society »La lettre du Collège de France [En ligne], 4 | 2008-2009, mis en ligne le 15 novembre 2010, consulté le 25 février 2024. URL : ; DOI :

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Henri Leridon

Emeritus Professor at the Institut national d’études démographiques

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