The evolution of trust

What you're seeing

Every coloured band is one strategy's share of the population, stacked to fill the whole height. Each generation, every player meets every kind of player and plays the dilemma for the chosen number of rounds; whoever earns the most points has more offspring next generation, whoever earns least dies back. So the bands are evolution in fast-forward — the strategies that thrive widen, the ones that flop pinch shut.

Start with Rounds per meeting at 1 and press play. With only a single encounter there's no future to protect, so being nice is pure loss — the red Cheater band swallows everything. That's the bleak baseline: in a one-shot world, betrayal is simply the better move. Now slide rounds up to ten or more and reset. The picture flips. Cheaters still feast at first on the naive Cooperators, but once the easy victims are gone they're left facing players who remember — the green Copycat (cooperate first, then do whatever you just did) and the amber Grudger (kind until crossed, then never again). Against those, cheating gets punished right back, and the cheats starve out. Trust wins not by being soft, but by being repeated and answerable.

That's the whole surprising lesson: cooperation doesn't need anyone to be a saint. It just needs the same players to keep meeting, to remember, and to return like for like. Nudge the Mistakes slider and watch how a little noise — fingers slip, signals get crossed — shakes the outcome, since now even the well-meaning occasionally wrong each other.

Also in Strategy & computation: Sorting algorithms →

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