For most of human history, coupling up was not merely a norm; it was a necessity. Before reliable contraception, women could not control their fertility, and most were far too poor to raise children alone. Hence the centuries-old convention that, whereas a tragic play or saga ends in death, a happy one ends in marriage. So the speed with which the norm of marriage—indeed, of relationships of any sort—is being abandoned is startling. Throughout the rich world, singlehood is on the rise. Among Americans aged 25-34, the proportion living without a spouse or partner has doubled in five decades, to...