**Introduction to Microeconomics** Game theory Chapter 9

**Elements of a Game** • Basic elements • The players. • The strategies. • The payoffs. • Payoff matrix • A table that describes the payoffs in a game for each possible combination of strategies. LO1: Basic Elements of A Game

**Strategy** Dominant strategy • One that yields the highest payoff no matter what the other players in the game choose Dominated strategy • Any other strategy available to a player that has a dominant strategy

**Prisoner’s Dilemma** • The Prisoner’s Dilemma • A classic example of potential conflict between the narrow self-interest of individuals and the broader interest of larger communities. • The Prisoner’s Dilemma • Each player has a dominant strategy. • The dilemma is: Payoffs are smaller than they would be if each player had played a dominated strategy. Lo4: The Effect of Dominant Strategy

**The Payoff Matrix for the original Prisoner’s Dilemma** • Will the prisoners confess? • Two prisoners, Horace and Jasper, are being held in separate cells for a serious crime that they did in fact commit. • The prosecutor, has only enough hard evidence to convict them of a minor offence. Lo4: The Effect of Dominant Strategy

**Table 9.3: The Payoff Matrix for the original Prisoner’s** Dilemma • Example 9.3: Will the prisoners confess? • The dominant strategy for each prisoner is to confess. Dominate strategy √ √ Lo4: The Effect of Dominant Strategy

**The Payoff Matrix for the original Prisoner’s Dilemma** • Will the prisoners confess? • The dominant strategy for each prisoner is to confess. Dominate strategy √ √ Lo4: The Effect of Dominant Strategy

**Table 9.3: The Payoff Matrix for the original Prisoner’s** Dilemma • Will the prisoners confess? • When each follows his dominant strategy and confesses, both will do worse than if each had shown restraint. Nash Equilibrium Better Outcome Lo4: The Effect of Dominant Strategy

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**Terminology** When a player tries to choose the "best" strategy among a multitude of options, that player may compare two strategies A and B to see which one is better. The result of the comparison is one of: • B dominates A: choosing B always gives as good as or a better outcome than choosing A. There are 2 possibilities: • B strictly dominates A: choosing B always gives a better outcome than choosing A, no matter what the other player(s) do. • B weakly dominates A: There is at least one set of opponents' action for which B is superior, and all other sets of opponents' actions give B at least the same payoff as A. • B and A are intransitive: B neither dominates, nor is dominated by, A. Choosing A is better in some cases, while choosing B is better in other cases, depending on exactly how the opponent chooses to play. For example, B is "throw rock" while A is "throw scissors" in Rock, Paper, Scissors. • B is dominated by A: choosing B never gives a better outcome than choosing A, no matter what the other player(s) do. There are 2 possibilities: • B is weakly dominated by A: There is at least one set of opponents' actions for which B gives a worse outcome than A, while all other sets of opponents' actions give A at least the same payoff as B. (Strategy A weakly dominates B). • B is strictly dominated by A: choosing B always gives a worse outcome than choosing A, no matter what the other player(s) do. (Strategy A strictly dominates B). • This notion can be generalized beyond the comparison of two strategies. • Strategy B is strictly dominant if strategy B strictly dominates every other possible strategy. • Strategy B is weakly dominant if strategy B dominates all other strategies, but some are only weakly dominated. • Strategy B is strictly dominated if some other strategy exists that strictly dominates B. • Strategy B is weakly dominated if some other strategy exists that weakly dominates B. Source: Wikipedia

**ThePrisonersDilemma.cdf**

**Will Pepsi spend more money on advertising?** • Imagine that Pepsi and Coca Cola are the only makers of cola drinks. Both are earning economic profits of $6000/day. • Assume the following: • If Pepsi increases its advertising expenditures by $1000/day and Coca Cola spends no more on advertising , Pepsi’s profit will increase to $8000/day and Coca Cola’s will decrease to $2000. • If both spend $1000 on advertising, each will earn an economic profit of $5500/day. • If Pepsi stands pat while Coca Cola increases its spending by $1000, Pepsi’s economic profit will fall to $2000/day, and Coca Cola’s will increase to $8000. • The payoffs are symmetric. LO1: Basic Elements of A Game

**The Payoff Matrix for an Advertising Game** LO1: Basic Elements of A Game

**Table 9.1: The Payoff Matrix for an Advertising Game** Suppose Coca Cola assumes that Pepsi will raise its spending on advertising, in that case, Coca Cola’s best option would be to follow suit. Payoff is higher LO1: Basic Elements of A Game

**Table 9.1: The Payoff Matrix for an Advertising Game** Suppose Coca Cola assumes that Pepsi will do nothing, in that case, Coca Cola’s best option would be to raise its spending on advertisements. Payoff is higher LO1: Basic Elements of A Game

**Table 9.1: The Payoff Matrix for an Advertising Game** No matter which strategy Pepsi chooses, Coca Cola will earn a higher economic profit by increasing its spending on advertising. Since this game is perfectly symmetric, a similar conclusion holds for Pepsi: No matter which strategy Coca Cola chooses, Pepsi will do better by increasing its spending on advertisements. Dominate strategy Nash equilibrium LO2: Identifying Dominant Strategy LO3: Find an Equilibrium for a Game

**Strategies** • Dominant strategy: • A strategy that yields a higher payoff no matter what the other players in a game choose. • Dominated strategy: • Any other strategy available to a player who has a dominant strategy. • Nash Equilibrium: • Any combination of strategies in which each player’s strategy is his best choice, given the other players’ strategies. LO2: Identifying Dominant Strategy

**Example 9.2: The Payoff Matrix for an Advertising Game When** One Player Lacks a Dominant Strategy No matter what Pepsi does, Coca Cola will do better to increase its advertising, so raising the advertising budget is a dominant strategy for Coca Cola. Dominate strategy Payoff is higher Payoff is higher LO3: Find an Equilibrium for a Game

**Example 9.2: The Payoff Matrix for an Advertising Game When** One Player Lacks a Dominant Strategy Pepsi does not have a dominate strategy in this game. Payoff is higher Payoff is higher LO3: Find an Equilibrium for a Game

**Example 9.2: The Payoff Matrix for an Advertising Game When** One Player Lacks a Dominant Strategy Nash equilibrium: If Pepsi believes that Coca Cola will spend more on advertisements, Pepsi’s best strategy is to keep its own spending constant. Dominate strategy Nash Equilibrium LO3: Find an Equilibrium for a Game

**Cartels** • Cartel: • A coalition of firms that agree to restrict output for the purpose of earning an economic (excess) profit. • Normally cartels involve several firms. • This makes retaliation against a dissenter difficult. • Agreements are not legally enforceable and hence may be unstable. • Constant temptation for each participant to cheat on the agreement. • Example: OPEC oil cartel production quotas. • Economic naturalist 9.1: Why might cartel agreements be unstable? LO5: Games with Equilibrium Like Prisoner’s Dilemma

**FIGURE 9.1: The Market Demand for Mineral Water** • Faced with the demand curve shown, a monopolist with zero marginal cost would produce 1000 bottles/day (the quantity at which marginal revenue equals zero) and sell them at a price of $1.00/bottle. D MR LO5: Games with Equilibrium Like Prisoner’s Dilemma

**FIGURE 9.2: The Temptation to Violate a Cartel Agreement** • By cutting its price from $1/bottle to $0.90/bottle, Aquapure can sell the entire market quantity demanded at that price, 1100 bottles/day, rather than half the monopoly quantity of 1000 bottles/day. 0.90 D MR 1100 LO5: Games with Equilibrium Like Prisoner’s Dilemma

**TABLE 9.4: The Payoff Matrix for a Cartel Agreement** • Each firm’s dominant strategy is to sell at the lower price, yet in following that strategy, each earns a lower profit than if each had sold at the higher price. Nash Equilibrium LO5: Games with Equilibrium Like Prisoner’s Dilemma

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