Critical Dialogue Diffusion of Democracy: The Past and Future of Global Democracy. By Barbara Wejnert. New York: Cambridge University Press, 2014. 363p. $99.00. doi:10.1017/S153759271500047X — Kurt Weyland, University of Texas at Austin During the last two decades, the social sciences have seen a dramatic increase in the study of diffusion processes; a growing number of scholars have investigated how and why economic, social, and political innovations spread across nations, or across states inside federal countries, such as the United States and Brazil. The first phase of research has examined the extent to which external influences impact domestic decision making: Do demonstration and contagion effects make a significant difference, and what is their effect, by contrast to the standard domestic determinants of decision making? After many scholars applying ever more sophisticated statistical techniques documented the importance of dif- fusion processes in a number of areas of society, politics, and policy, a second stage of research in the last few years began to disentangle the exact nature of external influences and the causal mechanisms through which they operate. Do decision makers learn from the success of front-runners and assess the benefits and costs of their reforms in systematic, thorough, and rational ways? Or do innovative experiences serve as models that exert strong normative appeal and raise the standards of appropriate behavior, pushing latecomers toward imitation? Alternatively, are innovations actively promoted by their initiators or by international organizations or great powers, which promote their spread with more or less forceful pressure? These questions, which mark the current research frontier, speak to major theoretical debates over rationality versus socio- logical motivations and over sovereignty and hierarchy in the contemporary global system. Consequently, the study of diffusion phenomena con- stitutes one of the most vibrant and exciting research areas in the social sciences. One added attraction arises from the fact that diffusion research productively trespasses outdated dividing lines between the subfields of comparative politics and international relations; between “American” politics and the remainder of political science; and, above all, among sociology, political science, and other disciplines. Barbara Wejnert’s wide-ranging and ambitious book embodies this interdisciplinary approach. As a political sociologist, she conducts a comprehensive analysis of the Diffusion of Democracy at the global level over the course of the last two centuries. While her predominant approach is the statistical investigation of a worldwide data set, the study also draws on the author’s intensive interview research with democratic activists in Eastern Europe, which focuses on the fall of communism and the subsequent transition to democracy. Chapter 1 sets up the analysis by explaining the mea- surement of democracy, which relies largely on Polity IV and Freedom House, and by depicting the growth of democracy in the world since 1800. Chapter 2 first offers an ample overview of the theoretical literature, which Wejnert divides into diffusion approaches and develop- ment approaches. She discusses a great variety of external influences, which can arise from the spatial proximity of democratic countries, from the density of democracy in a region of the globe, from networks of communication channels, and from the media, for instance. Internal factors, by contrast, include a country’s socioeconomic development level, its value system, and political conditions, especially prior experience with democracy. To capture the interplay of diffusion and development, Wejnert then designs a threshold model of democratic transition. Accordingly, particularly strong external influences can lift a country toward democracy at low development levels, while a high development level can produce this outcome despite limited diffusion effects; intermediate combinations of the two types of factors tend to have the same result. In the second half of Chapter 2, she illustrates the threshold model through an application to the downfall of communism in Eastern Europe and then provides thorough evidence of diffu- sion networks by reporting many interesting findings and quotes from her interviews with East European activists, with a special focus on her native Poland. Chapter 3 contains the analytical core of the book, namely, statistical analyses that assess the relative impact of development versus diffusion factors. To do justice to the complexity of external influences, which can operate at the country and regional level, Wejnert applies hierarchical modeling and complements her investigation of the global growth of democracy with studies of all the major regions 494 Perspectives on Politics © American Political Science Association 2015 Critical Dialogues