The simplest strategy won

A famous puzzle about trust, and the tournament that surprised everyone who entered.

Two people are arrested for a crime and questioned in separate rooms.

Each is offered the same deal. Stay silent, and if your partner stays silent too, you both get a short sentence. Betray your partner while they stay silent, and you walk free while they serve a long one. Betray each other, and you both serve a medium sentence.

You cannot talk to your partner. What do you do?

Work through it and betrayal looks like the smart move. Whatever the other person does, you personally come out ahead by betraying.

But if both people reason that way, both end up worse off than if they had trusted each other.

This is the prisoner's dilemma. Researchers at the RAND Corporation began studying the underlying game in 1950, and the mathematician Albert Tucker gave it the prison story that made it famous.

It describes far more than prisoners. Two companies deciding whether to slash prices. Two countries deciding whether to build more weapons. Two roommates deciding whether to do the dishes. Any situation where what is smart for each is bad for both.

Around 1980, a political scientist named Robert Axelrod asked a new question.

What if the game is played over and over, with the same partner?

He invited experts in game theory, economics, psychology, and mathematics to submit computer programs that would play a repeated prisoner's dilemma against each other in a tournament. Some strategies were elaborate. Some were sneaky. Some tried to detect weakness and exploit it.

The winner was the simplest program entered.

It was called Tit for Tat, submitted by the psychologist Anatol Rapoport. Its rules fit in a sentence. Cooperate on the first move. After that, do whatever the other player did last time.

Axelrod ran a second tournament. This time everyone knew Tit for Tat had won, and could design strategies to beat it. More than sixty entries came in.

Tit for Tat won again.

Here is the strange part. Tit for Tat never beats anyone in a single match. At best it ties. Its opponent always scores at least as well as it does.

It won the tournament anyway, by doing reasonably well against everyone, and by making cooperation easy for others to find.

Axelrod studied why. The strongest strategies shared a few traits.

They were nice, never the first to betray.

They were retaliatory, answering betrayal quickly, so they could not be exploited.

They were forgiving, returning to cooperation as soon as the other side did.

And they were clear, simple enough that others could understand them and adjust.

Being clever mattered less than being readable.

The one-time version of the dilemma, the puzzle in the interrogation rooms, says trust is a losing bet. The repeated version says something closer to real life. When people expect to meet again, cooperation stops looking naive and starts looking smart.

Axelrod had a phrase for what makes the difference: the shadow of the future.

Most of life is played in that shadow. The colleague you will work with next year. The neighbor. The client. The friend.

Start kind. Answer bad faith. Forgive quickly. Be easy to read.

"Trust men, and they will be true to you; treat them greatly, and they will show themselves great."

Ralph Waldo Emerson, "Prudence," 1841