Warren E. Miller and Donald E. Stokes's 1963 American Political Science Review article, “Constituency Influence in Congress” began a tradition of using public opinion survey data to analyze and understand the empirical basis of political representation in the modern democracy. Lawrence Ezrow's book is a continuation of that tradition. In it, Ezrow links contemporary empirical research on comparative political representation (e.g., Michael D. McDonald and Ian Budge's Elections, Parties, and Democracy: Conferring the Median Mandate, 2005) with more theoretical work on party systems, party strategy, and niche parties (e.g., Bonnie M. Meguid's Party Competition Between Unequals, 2005) to assess how electoral systems affect the nature of political representation in advanced democracies. Ezrow's central insight, and the main theme of his book, is that an electoral system does not directly structure the nature of representation in a polity as much as it alters the mixture of parties, hence policy options, that receive parliamentary representation. In particular, niche parties, that is, parties that adopt extreme left-wing or right-wing positions (p. 11), are far more likely to secure parliamentary representation under proportional representation electoral systems. This, he argues, is a good thing because niche parties deliver a fundamentally different style of representation than do mainstream, centrist parties.

The central datum of Ezrow's book is the 10-point left–right scale that is contained in repeated waves of the Eurobarometer survey. Readings on this scale provide the author with information on the ideological locations and movements of Western European electorates and political parties. He uses this information to establish four statistical results. The first is that Western European political parties are rewarded electorally for political moderation. More specifically, the closer a party's policy position is to the electorate's ideological center of gravity, the greater is the party's vote share—and this is true regardless of the electoral system in place. The second result is a null one, to wit, that there is no statistical evidence that proportional representation electoral systems encourage policy extremism or political polarization; parties in plurality electoral systems are at least as politically extreme in relative terms. Ezrow's third result is that niche parties are both more numerous and more successful (in terms of votes and seats) the more proportional the electoral system. In other words, the chief contribution of a country's electoral system is to limit or expand the electoral presence and parliamentary representation of niche parties. The final result is a bookend to the first: Whereas mainstream parties do better at the polls as they move closer to the political center, niche parties do better when they offer distinctly noncentrist policy positions that reflect the preferences of their core supporters.

The overall picture, in Ezrow's view, is therefore one of mainstream parties catering to the policy preferences of the mean voter in the general electorate, while niche parties cater to the policy positions of their noncentrist supporters. Thus, to the extent that disproportional electoral systems discourage the parliamentary representation of niche parties, they deprive their electorates of a set of distinctive, noncentrist policy options (p. 121).

Political theorists are unlikely to embrace the book's spare conception of political representation, that is, policy proximity as measured by a one-dimensional 10-point left–right scale, and there is room to debate whether the parliamentary presence of extreme views is in and of itself a good thing. Yet Ezrow's work is emblematic of quantitative, large-n work in comparative politics. The strength of that genre is not conceptual nuance or causal identification, but rather a capacity to harness vast amounts of cross-national data to illustrate broad political trends, patterns, and correlations. Judged on the genre's own terms, this book makes a notable contribution to the Miller-Stokes tradition, demonstrating the indirect role of the electoral system in shaping not just the size of the party system but also the mixture and type of parties that inhabit the party system and represent citizens, voters, and supporters.