Political parties shape the character of democratic governance, E. E. Schattschneider rightly told us long ago. Similarly, Votes from Seats presents an outstanding scholarly contribution telling us how two fundamental features of electoral systems shape the character of party systems. From the product of just two numbers, assembly size and average district magnitude, Matthew Shugart and Rein Taagepera deduce four power laws that predict, with remarkable accuracy, central tendencies of (1) the assembly seat share of the largest party, (2) the effective number of assembly parties, (3) the largest party’s vote share, and (4) the effective number of parties receiving votes. That the laws are deduced is the predicate for the second theme of the book. Shugart and Taagepera seek to arrive at an understanding of the basic contours of party systems in an iterative process of thinking, predicting (with quantitative specificity), observing, testing, rethinking, retesting, and so on, promoting boldly and convincingly a “social physics” approach that Taagepeara has been employing and honing since his articles in Social Science Research on seats—votes and assembly sizes in the early 1970s.

The authors start their logical model-building by asking what is possible and impossible; for example, the number of parties winning seats in a district must be at least one but never more than the district magnitude. They speculate that the geometric mean of the minimum and maximum possibilities could be a plausible expected value for the number of parties winning at least one seat in a district. In a nine-seat district, the square root of (1*9), the product of the possible number of minimum and maximum winning parties is 3. They check their expectations against reality; for example, one-seat districts have one winning party, four-seat districts average two winning parties, nine-seat districts average three winning parties, and so on. The expectations hold remarkably well, and so the thinking can turn to another party system feature. As a next step, to take one more example, Shugart and Taagepera reason that the average number of parties nationwide cannot be smaller than the average at the district level nor larger than the average if, or when, elected in a single nationwide district. By once again speculating that the geometric mean of those two bounds is a plausible expectation, their logical model indicates that the number of seat-winning parties of any size (N) equals the fourth root of the product of average district magnitude (M) and the assembly size (S). This expectation holds remarkably well, too.

That is some of the flavor of the core theses of the book, Chapters 7 through 10. Within that section, the reasoning and testing cover expectations for the effective number of vote-earning parties, the specifications and reasoning behind the four laws, and the encouragement or constraint that an assembly size places on the number of parties competing in the various districts. In the lead-up to developing ideas in that core section, the authors review and describe the dizzying possible variations on simple and complex electoral systems, being careful to help the reader by using case applications for a few selected countries to illustrate how various rules operate. They follow up on the core section with analyses of the conditioning effect of a presidential office on the number of parties (adding variability but not much affecting the central tendencies), of their models’ applicability to electoral systems that foster intraparty competition and use other complex rules (their models apply), and of whether a society’s ethnic diversity adds informational value to an understanding of the party system fragmentation (some, but not much).

Shugart and Taagepera are not looking for causes in a deterministic sense of necessary and sufficient conditions for how electoral rules shape the structure of party systems. Rather, the search is for an understanding of constraints that account for central tendencies. They are also not looking to produce a theory of party system structure. In 300-plus pages, they are almost totally successful in avoiding use of the word “theory.” Instead, they seek a set of logical models with connections that flow first from the preceding and then to the next and the next.

In what sense, then, is it proper to characterize Votes from Seats as an outstanding scholarly contribution? Are “cause—effect” and “theory” not the watchwords of empirical political science? The short answer is yes, but the longer response from Shugart and Taagepera is that building logical models of the sort they engage in producing is, possibly and likely, a richer way to get to understandings of cause and effect and cumulative theory building.

The authors’ models are well constructed and repeatedly shown to have strong empirical support for various central tendencies related to parties’ seats and votes. Extending outward from the models, the book offers evidence of linkages to, for example, government duration (in brief passing, pp. 108–9), a reconsideration of Maurice Duverger’s theses (pp. 114–20), and an intriguing but yet to be fully developed connection to disproportionality (pp. 141–47). What is equally important for empirical considerations of cause and effect, the models they have built will all but necessarily have to be called upon to serve as a baseline for further attempts to understand party structure. The book’s commentary keeps readers well aware that the models are about central tendencies around which there is a good deal of variation that requires explanation. The numerical precision of their logical model predictions make it easy to imagine that the next generation of party system scholarship will have a field day identifying conditions under which the precise predictions do and do not hold, much as previous generations have enjoyed picking apart the originally described cube law of vote-to-seat translations under first-past-the-post, single-member-district electoral systems. Numerically precise expectations serve as a handmaiden of progress in many social scientific inquires.

Votes from Seats provides an unstated and indirect challenge to the way that much of political science thinks empirical theory is built. Some suppose that because politics is a human construction, a well-developed theory needs to be built on the foundation stone of methodological individualism. Preferences molded by incentives give rise to choices, and thus it would be wise to build theories founded on preferences and incentives. Shugart and Taagepera offer a different path. They want to understand a set of macro-level party system outcomes where the important proximate forces operating as constraints on what is possible are a set of macro-level institutional features. With often no reference to individual thinking (and, once in a while, reliance on modest leverage from speculations about voters’ and party leaders’ thinking), a set of logically deduced and connected models of the typical macro-level outcomes is possible. The value provided to theory building is to know, with a good deal of confidence and a great deal of precision, what micro-level foundation stones need to be explored. As the authors remark, there is a good deal of politics to be explained reaching forward to other aspects of party systems and democratic representation at the macro level and reaching down to the micro level.

Scholars have much to ponder in the book’s wide-ranging treatment of party system structure and elements related to that structure. A graduate seminar could be organized around the ideas it is trying to teach and the ideas that likely extend from that teaching. Relying on it in undergraduate courses would be a stretch. Because Shugart and Taagepera are so thoroughly familiar with their subject matter, it appears to escape them at times that readers would be helped by words that would remind them of points that no doubt have become self-evident to the authors. They are also so thoroughly self-conscious of slight variations on electoral rules that they too often anticipate possible objections and too often announce a defensive posture that a potential objection is to be dealt with in a later section or chapter. For readers with more than the transient interest of one course in an undergraduate career, however, patient reading and rereading of Votes from Seats is going to produce influential insights for years to come.