At the beginning of human history, God gave mankind a mandate to “Be fruitful and increase in number; fill the earth” (Genesis 1:28). Sometime later—around the 19th-century—people started wondering, “Is the earth close to being filled with humans?”
In 1798, Thomas Malthus predicted that if current birth rates persisted, many in Great Britain would starve to death. Instead, the birth rate was matched by increased agricultural yields, allowing more people to be fed with fewer land resources.
Despite Malthus’s failed predictions, others worried that population would eventually outgrow our resources. In 1838, the Belgian mathematician Pierre Verhulst calculated that his country could never support more than 9.4 million people. Verhulst was wrong; Belgian’s current population is more than 11 million.
In 1925, Raymond Pearl, head statistician for the U.S. Food Administration during World War I, calculated the maximum population limit of the U.S. to be 200 million. We reached that in 1968 and are currently at around 319 million. Pearl also predicted the world population limit would be 2 billion, a number that was surpassed in 1930.
Other similar calculations and predictions followed—and they turned out to be just as faulty. Why do smart people get population limit predictions so wrong? As Adam Kucharski explains,
In 1995, mathematician Joel Cohen, at Rockefeller University in New York, tallied up dozens of global forecasts published to date, and found that they varied widely, from less than 1 billion to more than 1 trillion. Most early estimates were, like Pearl’s, far below 6 billion, the world’s population at the time.
According to Cohen, their flaw lay in the assumption that resource constraints, and hence carrying capacity, were fixed. In mathematical lingo, K was a constant: It never changed. This presumption, Cohen said, ignored human innovation. “Let us recognize, in the phrase of U.S. president [George H.W. Bush], that ‘every human being represents hands to work, and not just another mouth to feed’,” he wrote in the journal Science. “Additional people clear rocks from fields, build irrigation canals, discover ore deposits and antibiotics, and invent steam engines; they also clear-cut primary forests, contribute to the erosion of topsoil, and manufacture chlorofluorocarbons and plutonium. Additional people may increase savings or dilute and deplete capital; they may increase or decrease the human carrying capacity.”
This had been the missing ingredient in early population models: Humans don’t just extract from a fixed set of resources, but can create new resources through invention.
