Human societies, at all times and places, have organised themselves around the will to live with others, not alone. But not any more. During the past half-century, our species has embarked on a remarkable social experiment. For the first time in human history, great numbers of people – at all ages, in all places, of every political persuasion – have begun settling down as singletons. Until the second half of the last century, most of us married young and parted only at death. If death came early, we remarried quickly; if late, we moved in with family, or they with us. Now we marry later. We divorce, and stay single for years or decades. We survive our spouses, and do everything we can to avoid moving in with others – including our children. We cycle in and out of different living arrangements: alone, together, together, alone.