You Have Found The Prisoners' Dilemma

On each turn of the game, you and Serendip must choose, without knowing the other's choice, between cooperating with each other and trying to take advantage of each other.
Following every turn of the game, you and Serendip will each receive a certain number of gold coins, the number depending for each of you on the choices made by both of you.

The game will continue until the wizard tires of it, and includes this additional stipulation:

Your chances of surviving to play the next turn are closely related to the average number of coins you have received on previous turns. If this average on a given turn drops below a critical number (chosen in an unknown way by the wizard), some unknown but Inconceivably Foul Fate will befall you. The same is true for Serendip, of course, but since neither of you knows the critical number, neither of you has any choice but to try on each and every turn to maximize your own income.

The game is afoot. What should you do? Try it out. Can you find a successful strategy? A better one? A best one? Try keeping a record of not only your own score with each strategy but Serendip's too.

Ready to Play?