Many experiments have been done on the Prisoner's Dilemma, to try to gauge the normal human behavior in a prisoner's dilemma-type situation.

The Flood-Dresher Experiment was a prisoner's dilemma game run 100 times between 2 players. In this case, the game was unfair -- one of the players had an inherent advantage over the other player, but the payoff matrix layout was still a prisoner's dilemma. The following is the table used in the experiment. (In this case, the payoffs are positive, that is, each rational player seeks to maximize the value in the matrix cell he ends up in.)

 Player 2 Defect Cooperate Player 1 Cooperate -1, 2 0.5, 1 Defect 0, 0.5 1, -1

As in the Prisoner's Dilemma, both players are better off defecting. But when both defect, they do relatively poorly. On the other hand, if both choose their "worse" strategy consistently, they should both gain.

In the 100 trials, Player 1 chose to cooperate 68 times, and Player 2 78 times. Player 1 began the game expecting both players to defect. Player 2 realized the value of cooperation and started cooperating. Both players started cooperating after the first 10 or so moves, though Player 1 would defect on a regular basis, unhappy that his payoff wasn't as big as Player 2's. This in turn brought retaliation from Player 2, who would defect on the next move.

Each player kept a log of comments for each move. Some of those comments are quite amusing. "The stinker," writes Player 2 after Player 1's defection. "He's a shady character... A shiftless individual--opportunist, knave... He can't stand success."

The players' comments reflect their concern about the final few moves. Both seem to realize that it would make sense for both to defect on move 100, since no retaliation from the other player is possible. Player 1 worries about starting to defect earlier than Player 2 so that he has the advantage. As the game was played, both players cooperated on moves 83 through 98. On move 99, Player 1 defected, and on move 100, both defected.

It is clear that the long-term prospect of the game encouraged cooperation. Since the game was played multiple times, it became beneficial for both players to cooperate. On move 100, however, the game suddenly becomes a regular prisoner's dilemma, and both players defect, as game theory advocates they should (although if they both cooperated they would ensure themselves a gain of 0.5 points).

This reasoning is troubling though. Since both players must realize that they will both defect on move 100, move 100 does not have to figure into the game. It can then be thought that move 99 is really the last move in the game, since both players are obviously going to defect on move 100. But if move 99 is the last move, both players should defect, since no retaliation is possible (both players will defect anyway on move 100, no matter what the other player did on move 99). So both players should defect on move 99 as well. Then, move 98 can be thought of as the last move in the game. This line of reasoning can be extended indefinitely until move 1. So should both players always defect?

Clearly not, since if they both cooperate, they will gain more than if both defect.

This paradox is still unresolved. William Poundstone, in Prisoner's Dilemma, says that "Both Flood and Dresher say they initially hoped that someone...would 'resolve' the prisoner's dilemma. They expected...someone to ...come up with a new and better theory of non-zero-sum games...The solution never came...The prisoner's dilemma remains a negative result--a demonstration of what's wrong with theory, and indeed, the world."