The late 18th century English economist Thomas Malthus – one of the first to express concern about overpopulation – observed that there should be no more people in a country than can “daily enjoy a glass of wine and piece of beef for dinner.” But what if people choose tofu and beer instead?

The contemporary “ecological footprint” model offers another way to think about the Earth’s carrying capacity.

“If everyone on the planet enjoyed the lifestyle of an average U.S. citizen, the Earth could support only one to two billion people,” says Naughton. “But if everyone used resources at the rate of the average African citizen, far more people could be supported.”

Joel Cohen, a prominent population researcher at Rockefeller and Columbia universities, offers a more nuanced perspective: “He points out that there is no ultimate answer to this question,” says Naughton. “Rather, we need to think about broader issues of equity and sustainability when we consider human numbers and environmental impacts.”