Thompson’s prediction was purely theoretical, but it became foundational to the field of demography. When the United Nations Population Division began projecting world population in the 1960s, its estimates were based on Thompson’s assumption that fertility rates across the world would converge to replacement level, further cementing the concept into demographic thought. This idea was attractive to politicians and policymakers since it meant they wouldn’t have to worry about populations growing or shrinking indefinitely. They could count on new generations of workers and taxpayers to keep the economy humming and government programs funded when older people stopped working. As it turned out, Thompson was only partially correct. Birth rates do decline as countries industrialize, but they don’t stop falling when they reach replacement level. In fact, fertility rates are now below replacement level in every post-industrial society, and it’s not clear where they’ll settle. But just because America’s fertility rate isn’t going to return to that 2.1 number anytime soon doesn’t mean that the fabric of society is going to collapse. In fact, low fertility poses some advantages: easing ecological pressures, preventing overcrowding and reducing the infrastructure costs that come with a growing population.