Life’s strongest storylines are punctuated by and enmeshed with other people. The principle of “linked lives”—that the lives of individuals affect and are affected by the lives of others—is repeated as a mantra in life course literature. And yet this stands in direct contrast to the state of research, which largely treats individuals as if they exist in isolation of others. This author’s wish for the study of human development is to take more seriously the interdependence of lives. To say that lives are “linked” says nothing about their nature, length, purposes, or consequences. The author illustrates the significance of social relationships in time—probing the beginnings, middles, and ends of relationships—and in shaping the life course. It is the author’s hope that developmental science will get better at revealing the intensity and complexity of human life as a social experience.

This article highlights how much adult life has changed in the decades since the publication of landmark works on adult development and explores what these great shakeups mean for understanding adulthood today. It points to the need for renewed attention to what is unique and what is shared in different life periods, the changing significance of age, the growing uncertainties and certainties of modern life, the expanding demands and dwindling protections of jobs, the fragility of marriage, and the need to rearchitect social institutions and policies. The new terrain of adult life demands new responses from governments, families, and individuals—as well as from scientists who must treat it in ways that do right by its contemporary complexities and realities.