This article introduces a theory of Nested Games which accounts for the cohesion of coalitions. The parties in a coalition are considered to be playing a game with variable payoffs. The payoffs depend on a higher-order game between the coalition and its opponents. Several political situations approximate to this conceptualization, such as Government and Opposition coalitions, factions inside parties, international coalitions, class conflict. The theory of Nested Games predicts the cohesion of coalitions as a function of the relative size of both the coalitions and the partners within each coalition.
The test case of the theory is the cohesion of French electoral coalitions in 1978. Empirical results corroborate the theory. All parties behave according to its predictions. Moreover, a difference in the way parties behave, according to whether the game is visible (by the electorate) or invisible, is discovered and explained.