“The US is an exception in the rich world in that its population is projected to keep growing through the 21st century, reaching some 421 million by 2100. But that’s much less a function of fertility — US fertility has been below replacement level for years — than it is of the country’s openness to immigration. Recent census projections show that if immigration to the US stopped tomorrow, the US population would begin to fall immediately and hit just 226 million by 2100.”