Why has the number of births declined everywhere, all at once? Some blame technology, particularly smartphones and social media. Others blame a kind of 21st-century weltschmerz—a sadness about the state of the world and our uncertain future in it. A long essay in The New York Times by Anna Louie Sussman, titled “Why So Few Babies? We Might Have Overlooked the Biggest Reason of All,” argues that today’s generation is too anxious about the future to make the irreversible commitment of having a child. So who is right? Is this about phones and technology, or is it a reflection of modern anxiety about the world? Or, perhaps, both? Birth rates have been declining in developed countries for a long time, as child mortality has declined, as women’s education has increased, as female labor-force participation has soared, as contraception use has proliferated, and as modern notions of feminism have empowered women to take more control over their bodies and their economic futures. And birth rates have continued to decline as smartphone usage has surged, as housing …