The developmental paradigm, reading history sideways, and cross-cultural data have converged to exert a profound influence on social scientists and ordinary people. Through the use of these tools, social scientists of the 1700s and 1800s concluded that family patterns in northwest Europe had undergone many substantial changes before the early 1800s. These conclusions were accepted until the last several decades of the 1900s, when almost all were seriously challenged; many were declared to be myths. Further, the developmental paradigm, reading history sideways, and the conclusions of generations of social scientists created a package of ideas—developmental idealism—that subsequently became a powerful influence for family change in many parts of the world during the past two centuries. This developmental idealism has been a substantial force for changing living arrangements, marriage, divorce, gender relations, intergenerational relationships, and fertility.