The birthrate in the United States, and in many other countries around the world, has been declining for decades. The US birthrate now is at 1.8, which is below the need for a stable population—the replacement rate of 2.1 children per woman. Yet the profound consequences of this change take generations to play out. In this Conversation, Jonathan V. Last, author of What To Expect When No One's Expecting and Executive Editor of The Bulwark, shares his research into demographic decline and spells out some of the current and potential future consequences that might follow from it. Last argues that we're likely to see a steep fall in the global population beginning later this century, which could produce alarming geopolitical dangers. As for America, Last notes that declining fertility could put major pressure on the social safety net—and change the nature of our society. This is a sobering and important Conversation on a neglected but extremely important subject.