Matching pennies
Matching pennies is a classic two-player, zero-sum game in game theory, in which each participant simultaneously selects to display either the heads or tails side of a penny.[1] The player designated as the matcher (typically Player 1) wins one unit from the other player if the choices match (both heads or both tails), while the mismatcher (Player 2) wins one unit if they differ.[2] This simple setup results in a symmetric payoff matrix where outcomes are +1 for the winner and -1 for the loser in each scenario: heads-heads and tails-tails yield (1, -1), while heads-tails and tails-heads yield (-1, 1).[3] The game has no pure strategy Nash equilibrium, as any predictable choice by one player can be exploited by the other to guarantee a win, highlighting the need for randomization in optimal play.[1] Instead, the unique mixed strategy Nash equilibrium requires both players to randomize their choices equally, selecting heads or tails with 50% probability each, which yields an expected payoff of zero for both and ensures neither can unilaterally improve their outcome.[2] This equilibrium is derived by setting the expected utilities equal for each pure strategy, making the opponent indifferent and preventing exploitation.[3] Matching pennies serves as a foundational example for illustrating key concepts in non-cooperative game theory, such as strategic interdependence, the limitations of deterministic strategies, and the role of mixed strategies in resolving games without dominant actions.[1] It demonstrates how rational players maximize expected payoffs in purely competitive settings and has been extended to variants with asymmetric payoffs or multiple rounds to model real-world scenarios like market competition or evolutionary biology.[2] The game's impartial and symmetric nature makes it ideal for teaching the minimax theorem and the value of a game in zero-sum contexts.[3]Game Description
Basic Rules
Matching pennies is a two-player, zero-sum game in which one player, typically designated as the matcher (or Player 1), aims to match the choice of the other player, known as the mismatcher (or Player 2), who seeks to avoid the match.[4] Each player simultaneously selects one of two options—heads (H) or tails (T)—by secretly positioning a penny or an equivalent token, without any communication between them.[5] Upon simultaneous reveal, the matcher wins and receives +1 payoff if both players choose the same side (both H or both T), while the mismatcher wins and receives +1 payoff if the choices differ; in each case, the losing player receives -1.[6] This structure ensures the game is strictly zero-sum, as the total payoff across both players always sums to zero.[5] Originating as a simple parlor game long before the formal development of game theory, matching pennies exemplifies basic simultaneous-move decision-making under uncertainty.[7]Payoff Matrix
The payoff structure of Matching Pennies is represented by a 2×2 bimatrix, where rows correspond to Player 1's actions (Heads or Tails), columns to Player 2's actions (Heads or Tails), and each cell contains the payoffs for both players in the form (payoff to Player 1, payoff to Player 2).[8][5] This matrix quantifies the outcomes from the basic rules: Player 1 wins (+1) and Player 2 loses (-1) if both choose Heads or both choose Tails, while Player 1 loses (-1) and Player 2 wins (+1) otherwise.[8] The game is zero-sum, as the payoffs in each cell sum to zero, reflecting pure competition where one player's gain equals the other's loss.[5]| Player 2 \ Player 1 | Heads | Tails |
|---|---|---|
| Heads | (1, -1) | (-1, 1) |
| Tails | (-1, 1) | (1, -1) |