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224As a kid, you probably played tic-tac-toe with your friends or siblings. In this game, two players take turns claiming cells of a 3×3 matrix (a matrix with three rows and three columns). Player 1 claims a cell by writing an “X” in it; player 2 writes an “O” in each cell that he claims. A player can claim only one cell per turn. The game ends when all of the cells have been claimed or when one of the players has claimed three cells in a line (by claiming an entire row, column, or diagonal). If a player has claimed three cells in a line, then he wins and the other player loses. If all of the cells are claimed and neither player has a line, then the game is declared a tie. Figure 17.1 depicts the playing of a game in which player 1 wins.Tic-tac-toe is a game of skill—nothing in the game is left to chance (no coins are flipped, no dice rolled, etc.). It is also a game of perfect information, because players move sequentially and observe each other’s choices. Further, it FIGURE 17.1A game of tic-tac-toe.Tic-tac-toematrixRound 1(Player 1moves first)Round 2Round 3XXOXXORound 4(Player 2blocks a line)Round 5Round 6(Player 2blocks)Round 7(Player 1wins withdiagonal)XXOOXXXOXXOOXXOOXXOOXO17PARLOR GAMES
225Parlor Games is a finite game. These facts imply that the result on page 188 (Chapter 15) holds for tic-tac-toe: this game has a pure-strategy Nash equilibrium. What else do we know about tic-tac-toe? It is a two-player, strictly competitive game: either player 1 wins and player 2 loses, or player 2 wins and player 1 loses, or the play-ers tie. Thus, the result on page 149 (Chapter 12) also applies, meaning that each player’s Nash equilibrium strategy is a security strategy for him.Let’s review. One of the outcomes (1 wins, 2 wins, tie) occurs in a Nash equilibrium. Because the equilibrium strategies are security strategies, each player can guarantee himself this equilibrium outcome. For example, suppose “1 wins” is the equilibrium outcome. Then our mathematical analysis proves that player 1 has a winning strategy,which guarantees that he will win the game regardless of what his opponent does.On the other side of the table, player 2 has a strategy that guarantees at least a loss, regardless of what player 1 does. In this case, player 2’s security strategy is not very helpful to her, but player 1’s secu-rity strategy is quite helpful to him. The game is “solved” as long as player 1 is smart enough to calculate his security strategy.In fact, “1 wins” is not the equilibrium outcome of tic-tac-toe. The Nash equilibrium actually leads to the tie outcome. Thus, our mathematical analysis implies that each player has a strategy guaranteeing a tie. In other words, when smart, rational players engage in tic-tac-toe, the game always ends in a tie. You probably knew this already, because most people figure it out relatively early in life. As a matter of fact, discovering the solution to tic-tac-toe is one of the major stepping stones of childhood development. I am sure child psychologists put it