A MALTHUSIAN NEVER TIRES OF BEING:

After the passage of 47 years, Dr. Ehrlich offers little in the way of a mea culpa. Quite the contrary. Timetables for disaster like those he once offered have no significance, he told Retro Report, because to someone in his field they mean something "very, very different" from what they do to the average person. The end is still nigh, he asserted, and he stood unflinchingly by his 1960s insistence that population control was required, preferably through voluntary methods. But if need be, he said, he would endorse "various forms of coercion" like eliminating "tax benefits for having additional children." Allowing women to have as many babies as they wanted, he said, is akin to letting everyone "throw as much of their garbage into their neighbor's backyard as they want."

Dr. Ehrlich's ominous declarations cause head-shaking among some who were once his allies, people who four decades ago shared his fears about overpopulation. One of them is Stewart Brand, founding editor of the Whole Earth Catalog. On this topic, Mr. Brand may be deemed a Keynesian, in the sense of an observation often attributed to John Maynard Keynes: "When the facts change, I change my mind, sir. What do you do?" Mr. Brand's formulation for Retro Report was to ask, "How many years do you have to not have the world end" to reach a conclusion that "maybe it didn't end because that reason was wrong?"

One thing that happened on the road to doom was that the world figured out how to feed itself despite its rising numbers. No small measure of thanks belonged to Norman E. Borlaug, an American plant scientist whose breeding of high-yielding, disease-resistant crops led to the agricultural savior known as the Green Revolution. While shortages persisted in some regions, they were often more a function of government incompetence, corruption or civil strife than of an absolute lack of food.

Some preternaturally optimistic analysts concluded that humans would always find their way out of tough spots. Among them was Julian L. Simon, an economist who established himself as the anti-Ehrlich, arguing that "humanity's condition will improve in just about every material way." In 1997, a year before he died, Mr. Simon told Wired magazine that "whatever the rate of population growth is, historically it has been that the food supply increases at least as fast, if not faster."

Somewhere on the spectrum between Dr. Ehrlich the doomsayer and Mr. Simon the doomslayer (as Wired called him) lies Fred Pearce, a British writer who specializes in global population. His concern is not that the world has too many people. In fact, birthrates are now below long-term replacement levels, or nearly so, across much of Earth, not just in the industrialized West and Japan but also in India, China, much of Southeast Asia, Latin America -- just about everywhere except Africa, although even there the continentwide rates are declining. "Girls that are never born cannot have babies," Mr. Pearce wrote in a 2010 book, "The Coming Population Crash and Our Planet's Surprising Future" (Beacon Press).

Because of improved health standards, birthing many children is not the survival imperative for families that it once was. In cramped cities, large families are not the blessing they were in the agricultural past. And women in many societies are ever more independent, socially and economically; they no longer accept that their fate is to be endlessly pregnant. If anything, the worry in many countries is that their populations are aging and that national vitality is ebbing.

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