Professor Paradoxicus has invented a new game. A single card is dealt to each of two players. The cards are consecutive and come from a 9 card deck, each card bearing a distinct integer between 1 and 9. Each player then holds his card (number facing outward) against his forehead; ie, each player can see his opponent’s card, but not his own. The players are then asked whether they want to bet, and if both agree, they examine their own card, with the player who drew the lower card paying his opponent the amount on the higher card.

Professor Paradoxicus has invited three students to analyse the game.

Simplicimus notes that the game is symmetric and zero sum. He asserts that players will be indifferent as to whether or not to bet.

Optimisticus points out that whenever a player sees his opponent holding a 9 he can count on losing. On the other hand, Optimisticus reasons, if the opponent is holding any card N<9, then that player has a 50% chance of winning N+1 and a 50% chance of losing N. Optimisticus asserts that a player will quit when he sees his opponent holding a 9, and choose to play otherwise.

Finally, Sceptisimus expresses the view that a player will refuse to bet unless he sees his opponent holding a 1. He justifies his peculiar opinion as follows: Suppose a player sees his opponent holding a 9. He will certainly refuse to bet. Now suppose a player sees his opponent holding an 8. Clearly, he himself will be holding either a 9 or a 7. In the former case, he can be sure that his opponent will refuse to bet. He concludes that he has nothing to gain by agreeing to bet, but could have something to lose. He therefore should refuse to bet.

Sceptisimus next reapplies this argument repeatedly, with 7 (,6,5..) taking the role of 8 (,7,6..).

What’s your view of these three arguments and how do things change if the players, while still aware that the deck is finite, don’t know what are the lowest and highest numbers?

(In reply to

re: Zero Sum Confusion by Dej Mar)

Dej Mar:

Sorry if I offended. It was certainly not my intention. However, I stand by the definition of zero-sum.

It is simply not true that "If the game is non-zero sum between two players, one of the players MUST have an advantage over the other." Zero sum games are by definition Win-lose (if I lose, you win by exactly the same amount, and vice-versa). But non-zero sum games can be cooperative win-win games, where rational players both have something to gain.

Here's a real-life non-zero sum game: I have a shovel and you have a map. If we both choose to play, then we share buried treasure. If either one of us chooses not to play, then we both get nothing. Before we find the treasure, we have to negotiate the split. If rational, we will both play. Probably, the player with the map will get a larger share of the treasure, but it depends on how well the two players negotiate.

I think what you perhaps mean to say is that if a game is zero-sum, then two rational players will play only if the expected payoff to each player is zero dollars. Otherwise, one of the players has a negative expected payoff and will decline to play.

But being zero sum and having an expected zero payoff are two very different things. Symetric zero sum games always have a zero payoff, but non-symettric zero sum games can favor one player over another.