Re-formalizing PD

by cousin_it 1 min read28th Apr 200963 comments


The Prisoner's Dilemma has been discussed to death here on OB/LW, right? Well, here's a couple new twists to somewhat... uh... expand the discussion.

Warning: programming and math ahead.


Scenario 1

Imagine a PD tournament between programs that can read each other's source code. In every match, player A receives the source code of player B as an argument, and vice versa. Matches are one-shot, not iterated.

In this situation it's possible to write a program that's much better than "always defect". Yes, in an ordinary programming language like C or Python, no futuristic superintelligent oracles required. No, Rice's theorem doesn't cause any problems.

Here's an outline of the program:

 // begin PREFIX
Strategy main(SourceCode other)
    // get source code of this program from "begin PREFIX" to "end PREFIX",
    // using ordinary quine (self-printing program) techniques
    String PREFIX = "..."

    if (other.beginsWith(PREFIX))
        return Strategy.COOPERATE;
        return anythingElse(other);
// end PREFIX

// from this point you can write anything you wish
Strategy anythingElse(SourceCode other)
    return Strategy.DEFECT;

Some features of this program:

  • It cooperates with itself.
  • It cooperates with any other program that begins with PREFIX.
  • It's impossible to cheat, because opponents that begin with PREFIX can't not cooperate with this program.
  • Other authors now have an incentive to include PREFIX in their programs, moving their original logic into the "anythingElse" subroutine. This modification has no downside.

So, introducing such a program into the tournament should lead to a chain reaction until everyone cooperates. Unless I've missed something. What say ye?

Edit: the last point and the conclusion were wrong. Thanks to Warrigal for pointing this out.


Scenario 2

Now imagine another tournament where programs can't read each other's source code, but are instead given access to a perfect simulator. So programs now look like this:

Strategy main(ObjectCode self, ObjectCode other, Simulator simulator) {...}

and can call simulator.simulate(ObjectCode a, ObjectCode b) arbitrarily many times with any arguments. To give players a chance to avoid bottomless recursion, we also make available a random number generator.

Problem: in this setting, is it possible to write a program that's better than "always defect"?

The most general form of a reasonable program I can imagine at the moment is a centipede:

  1. Programmer invents a number N and a sequence of real numbers 0 < p1 < p2 < ... < pN < 1.
  2. Program generates a random number 0 < r < 1.
  3. If r < p1, cooperate.
  4. Simulate the opponent's reaction to you.
  5. If opponent defects, defect.
  6. Otherwise if r < p2, cooperate.
  7. And so on until N.

Exercise 1: when (for what N and pi) does this program cooperate against itself? (To cooperate, the recursive tree of simulations must terminate with probability one.)

Exercise 2: when does this program win against a simple randomizing opponent?

Exercise 3: what's the connection between the first two exercises, and does it imply any general theorem?



Ordinary humans playing the PD othen rely on assumptions about their opponent. They may consider certain invariant properties of their opponent, like altruism, or run mental simulations. Such wetware processes are inherently hard to model, but even a half-hearted attempt brings out startling and rigorous formalizations instead of our usual vague intuitions about game theory.

Is this direction of inquiry fruitful?

What do you think?