The Theory of Political Coalitions

Riker argues that political coalitions follow a size principle: in zero-sum games, winning coalitions tend toward the minimum size necessary to win, because each member wants to maximize their share of the payoff; in non-zero-sum games, they tend toward the minimum winning size required to secure the benefit. He rejects descriptive, impressionistic accounts of coalition behavior and the assumption that politicians build oversized coalitions for legitimacy or safety. His model treats politics as strategic, rational, and payoff-oriented, with coalition size determined by members' calculation of advantage.

Riker introduced the size principle—that winning coalitions tend toward minimum winning size—as a general proposition about political coalitions. It became a foundational concept in rational-choice political science and is applied to legislative coalitions, parliamentary government formation, party systems, and international alliances. It also informed Riker's later work on federalism as a bargain and on heresthetics—the art of manipulating political dimensions to win. The size principle remains a standard reference in coalition theory, though empirical tests have qualified its scope.

The Theory of Political Coalitions provides the game-theoretic foundation for Riker's later Federalism: Origin, Operation, Significance. There, federal unions are treated as coalitions formed by elites seeking expansion or security, and the federal bargain is the exchange of autonomy for union. The size principle explains why federal bargains take the form they do: leaders seek the minimum concessions necessary to secure the union. Riker's federalism theory is essentially the coalition model applied to territorial politics, which is why the two books are usually read together.

The size principle has been influential but contested. Studies of parliamentary coalitions have found many minimum-winning cabinets, but also oversized and minority governments, which the theory struggles to explain. Legislative coalition research shows that minimum-winning behavior is common but not universal, and that ideological proximity, institutional rules, and policy preferences also shape coalition size. Riker himself qualified the principle for non-zero-sum settings. It survives as a powerful baseline model rather than a universal law of coalition formation.

Riker's framework is used in parliamentary government formation, where parties calculate the minimum winning coalition; in legislative bargaining and logrolling in the U.S. Congress; in international alliance formation and burden-sharing; and in coalition-building in multi-party democracies. It also underpins formal models of cabinet stability and party competition. The size principle and its elaborations are standard tools in positive political theory and comparative politics, especially in European coalition studies and in research on coalition governments.

A common misconception is that Riker claims all coalitions are minimum-winning. He qualifies the principle by distinguishing zero-sum from non-zero-sum games and treats it as a tendency, not a universal law. Another misreading treats the book as a work of federalism; it is a general rational-choice theory of coalitions, and its federalism relevance is indirect, through Riker's later application. Readers looking for a theory of federal institutions will not find it here—they will find the strategic logic that Riker later applied to federal bargains.