In the absence of reliable statistics theologians, philosophers and other writers continued to theorize freely on demographic issues until the early 19th century. The writings of Thomas Malthus in the late 18th and early 19th century mark a new level of the debate. The British economist and cleric held that population growth, epidemics aside, was always far faster than the growth of food production, and that this would lead to overpopulation and famine. The choice was between doing nothing, and facing the painful consequences as nature eliminated the excess of population, or controlling birthrates, beginning with the suspension of all aid to the poor so as to make them “responsible” — choosing to marry and procreate only if they had the means to feed and educate children. Malthus believed the spread of poverty was a threat to all humanity.

The French philosopher and socialist Pierre-Joseph Proudhon believed there was no overpopulation problem: “There is only one man too many on earth, and that is Mr. Malthus.” Poverty spreads, he said, because of the system of property, which gives some unfair power over others. Karl Marx rejected Malthus’s belief in a “principle of population,” a universal natural law whereby population growth was always faster than the growth of resources. What mattered, Marx wrote, was not the size of the population but the distribution of wealth.

The debate hardly went further until the mid-20th century, when the world population doubled from 3 billion in 1950 to 6.1 billion in 2000. Demographers, economists, philosophers, historians and politicians disagreed fiercely over the interpretation of this surge. Some advocated the proliferation of life, regardless of quality; the realists stressed the need to control procreation. Populationists claimed it was a matter of unequal development rather than actual demographic growth; modern Malthusians retorted that this attitude would condemn hundreds of millions to starve. From the 1980s the debate included environmental and ecological issues.

Today the anti-Malthusians point to signs of change: Fertility rates are crashing, even in poor countries. This confirms the “demographic revolution” referred to in 1934 by the French politician Adolphe Landry, who argued that a increased food production had made the problem of the relationship between population and resources irrelevant.

It has been suggested that the global population will stabilize at 9 billion in 2050 and 10 billion in 2150. The majority of demographers argue that since the Earth is capable of feeding 10 billion it cannot be overpopulated at only 7 billion, and that if 1 billion people are suffering from undernourishment and 2 billion are living in poverty, it is because resources are badly distributed. But is it desirable for the global population to grow so large?

In 1997 Salman Rushdie wrote to the six billionth world citizen, due to be born that year: “It has proved impossible, in many parts of the world, to prevent the human race’s numbers from swelling alarmingly. Blame the overcrowded planet at least partly on the misguidedness of the race’s spiritual guides. In your own lifetime, you may well witness the arrival of the nine billionth world citizen. (If too many people are being born as a result, in part, of religious strictures against birth control, then too many people are also dying because [of] religious culture.)” In 2011, or early 2012 at the latest, we are expecting the arrival of the 7 billionth world citizen. He or she has a 70 percent chance of being born into a disadvantaged family in a poor country. Should we be preparing a welcome or an apology?

Georges Minois is a historian and the author, most recently, of “Weight of numbers: the historical obsession with overpopulation.” This article was translated from the French by Charles Goulden.