The political science world today is filled with “must-read” books, and so it is with trepidation that I seek to add another tome to the list. Nonetheless, this new study by Robert Huckfeldt, Paul Johnson, and John Sprague addresses an important problem and does so with such innovative and well-executed theory and data that I have no doubt whatsoever that the book deserves such recognition; indeed, it should is a must-read for all social scientists interested in how democracies can be sustained. This is a bold claim, so let me explain.

The problem the authors address is one of the most important challenges to democracy today: the need to keep discussions among competing political viewpoints alive. The marketplace of ideas in all democracies is threatened by homogeneity (of a compartmentalized sort), the cornerstone of which is strong (or “bonding”) network ties. Unless some mechanisms can be identified by which weak ties (“bridging” ties) can be created across extended and diverse social networks, the vitality of the discussion essential to democracy is at risk.

The research reported in the book is extraordinarily innovative. First, the authors conduct a statistical analysis on what seems to be one of the best social networks surveys ever conducted in political science (in part because the authors interview not only individual respondents but also other network numbers named by the respondent). Many innovations can be found in the analysis, including an effort to measure the accessibility of information other members of the social network (via the speed of responses), and an effort to incorporate campaign dynamics into their equations. Throughout, the reader cannot help but be impressed with the great care and insight with which various processes are disassembled and then rendered into microlevel concepts and hypotheses tested through rigorous analysis.

Many challenges to conventional wisdom emerge from this portion of the book. Disagreement seems not to produce dissonance for citizens, leading to the avoidance of political material (even if this is so because many people do not care enough to be alarmed). Nor is political disagreement inevitably associated with shunning. The authors' conclusion challenges conventional wisdom: “In short, political differences and disagreements are easily reconciled, readily accommodated, and simply explained by many citizens, and hence they are unlikely to be dissonance producing” (p. 96). And while debate will surely surround their claim that unexpectedly widespread network heterogeneity characterizes discussion groups in the United States today, the authors develop an important enigma: More network diversity seems to exist than would be expected on the basis of theories of network homogeneity, political persuasion, and Elizabeth Noelle-Neumann's “spiral of silence.”

Yet Political Disagreement is more than the sophisticated statistical analysis that we have come to expect from the reports of Huckfeldt and his colleagues. Beginning with Chapter 6, the authors employ agent-based models in an attempt to capture the dynamics of network homogeneity. The modeling effort is designed to “ascertain the aggregate impact of various individual strategies, behavioral rules, and responses” (p. 128).

Chapter 6 presents network analysis that is based on what I consider to be an entirely implausible assumption about persuasion. Chapter 7 is vastly more interesting, owing to the use of a different assumption about persuasion under the instance of heterogeneity. The authors rely on the idea of an autoregressive process—“people may understand political messages within a context created by their own preferences as well as by the preferences of others” (p. 74). The driving assumption in this model is that people change their political views only in conjunction with (or in interaction with) the views of those in their social networks. In short, “people respond [to diversity in their environments] by comparing new and divergent opinions against the opinions of others within their communications network,” and individuals become evaluators “of information received through a successive autoregressive process of social interaction” (p. 161). With this (and a few other) assumptions, the authors are at last able to produce an interesting model consistent with the empirically based view that network homogeneity does in fact exist.

Of course, one can still quibble with the assumptions of the model, and especially the inattention to individual differences, but one cannot help but be impressed with the care and insight with which the model was developed—as well as the tenacity of the authors in tweaking the model until the benchmark outcomes were produced.

The overall conclusions of the book are uncharacteristically optimistic for a study of mass political opinion and behavior. Democracy can be sustained so long as network ties remain weak (low density networks) and political minorities do not become isolated. Few network analysts are so sanguine about the ability of citizen interactions to contribute to the democratic marketplace of ideas.

Of course, as good as it is, this is not a perfect book. The writing, for instance, is very redundant. One wonders about some methodological details, especially with regard to sampling and the complexities (which are substantial) of network analysis. Perhaps the most important substantive omission of the book—which is a natural consequence, given the focus on networks—is the inattention to the socially atomized (which is about 20%–25% of the American people). These folks are not just interesting because they may be relevant to politics and since we understand little about how they form their preferences. They may also perform an important function related to the homogeneity of social networks. Since atomization likely changes over one's life span, when atomized individuals join social networks, they can perform a useful role in upsetting equilibria. More generally, further thinking about social networks would profit from conceptualizing the process in more dynamic terms. Consider the very important role of marriage and divorce. Individuals get married and join social networks, which have a tendency toward homogeneity. Then along comes divorce, rendering some individuals in a state of atomization for a time. Then recoupling occurs, with the strong possibility of joining a new social network, in which one might well hold minority viewpoints. And important gender differences undoubtedly exist in these processes. I admit that these dynamics may be difficult to study and model, but they are important to analyze.

Finally, the primary, if not exclusive, focus of this book is on electoral campaigns. One wonders whether many of the findings reported here extend to other, more threatening and dangerous types of political discussion, as in topics of political tolerance and intolerance. From that literature, we know many things that seem to be at odds with the authors's assumptions and findings (e.g., it is easier to talk tolerant people into an intolerant viewpoint than vice versa). Future scholarship will surely find it valuable to expand the model of this book to more incendiary forms of political discussion and interaction.

Huckfeldt, Johnson, and Sprague have not provided the last word on network homogeneity. Contrary views exist among network analysts on many important aspects of their inquiry. But no one interested in deliberative democracy can ignore this brilliant work by three gifted social scientists. Political Disagreement is a stellar exemplar of how social scientists should use theory to develop hypotheses and collect and analyze empirical evidence, while also developing formalized models consistent with and explicating the empirical evidence. The book is a profound contribution to our understanding of democracy and its continuing efforts to sustain the ever-so-crucial marketplace of ideas.