Prisoner’s Dilemma

INTRODUCTION

Imagine the following hypothetical situation: You and your partner in crime have been arrested for two robberies. The police are interrogating you and your partner in separate cells. They have substantial evidence placing you at the scene of the crime of the first, less serious, robbery, but they do not have sufficient evidence placing you at the second and more serious robbery. They have made a proposal to you: implicate your partner in the second robbery and you can get off with just probation. You know that your partner is being interrogated as well and that she may break. If she does, you could go to jail for a very long time.

Here are the consequences: If both you and your partner remain silent (cooperate with one another), you’ll both face light sentences of a year in jail each. If you both implicate one another (defect), you will both face a long jail sentence of 10 years each. If you cooperate and your partner defects, then you will face an even longer sentence of 20 years and she will get probation. Finally, if you implicate your partner and she keeps quiet, then she will face 20 years and you will just get probation.

What do you do in such a situation? Would it matter whether you will encounter your partner again?

This scenario, known as the prisoner’s dilemma, has been a staple in economic and human behavior studies for the past half century. It illustrates the problem of reciprocal altruism: cooperation would be in the best interests for both parties together, but individuals are tempted to defect (not cooperate) because defecting leads to a better outcome for the individual no matter what the other player does.

Non-human animals also face situations like the prisoner’s dilemma, in which cooperation would be the best strategy for the pair (or larger group) but defection would be best for the individual. How can cooperative behavior evolve in such a circumstance?

In this simulation you will play games of a form of prisoner’s dilemma against the computer. If you and the computer both cooperate, you each receive 10 points. If you both defect, you each receive no points. If you cooperate but the computer defects, then the computer receives 15 points and you receive minus 10 points. Conversely, if you defect and the computer cooperates, you receive 15 points and the computer receives minus 10 points. Keep in mind that the object is to score as many points as you can, not to beat the computer.

SIMULATION

QUESTIONS

Part I: Single Instance Prisoner’s Dilemma

Check the box that says “New Opponent each round.” To start the simulation, click on either the Cooperate or Defect button.

First, try ten bouts of playing cooperate. In each of ten bouts, hit the Cooperate button. Record your score as well as that of the computer program.

 

Question 1. What score did you receive? What score did the computer receive?

Now play ten bouts of playing Defect. In each of the ten bouts, hit the Defect button. Record your score as well as that of the computer program.

 

Question 2. What score did you receive? What score did the computer receive? Compare your scores to those for Question 1.

 

Question 3. Based on your answers to Questions 1 and 2, which is the better strategy: Cooperate or Defect?

 

Part II: Repeated Prisoner’s Dilemma

A critical feature of the early versions of the prisoner’s dilemma was that the players faced one another only once. Given that the opponents had no prospect of meeting up again, it did not pay to cooperate. What if players did play multiple bouts of a prisoner’s dilemma game? Would repeated rounds with the same players make cooperation a more profitable strategy?

To explore these questions, you will now play against the same opponent (played by the computer) for ten rounds. This player will have knowledge of the past moves you have made as well its own.

Check the box that says “Same Opponent each round.”

First, try ten bouts of playing Cooperate. In each of ten bouts, hit the Cooperate button. Record your score as well as that of the computer program.

 

Question 4. What score did you receive? What score did the computer receive?

Now play ten bouts of playing Defect. In each of the ten bouts, hit the Defect button. Record your score as well as that of the computer program.

 

Question 5. What score did you receive? What score did the computer receive? Compare your scores to those for Question 1?

 

Question 6. Based on your answers to Questions 4 and 5, which is the better strategy: Cooperate or Defect?

 

Question 7. Which strategies work best?

 

Question 8. Can you judge what strategy the computer is playing?