Tit-for-tat
Start by cooperating and from then on simply repeat whatever the other player did in the previous round.
Often misread asTaking its tournament success as proof that it is optimal or an equilibrium, when its performance depends on the mix of opponents it happened to face and it handles mistakes badly.
How to see it
Robust, not optimal, and badly hurt by noise
Open by cooperating, then do whatever the other side did last time. It never beats its opponent in any single encounter, and it does well over the long run because it draws cooperation out of anyone willing to offer it and refuses to be exploited twice.
The checkOne accidental defection against another tit for tat starts an alternating retaliation that neither can stop. It is robust rather than optimal, and noisy settings need some forgiveness built in.
Try itPlay two tit for tat players against each other, introduce one accidental defection, and trace the next six rounds. Count how many of them are mutual cooperation. The answer is none.
Know these first
Leads on to
Easily confused with
Sources
- Axelrod, R. (1984). Basic Books. Source of 'the shadow of the future'.
- Axelrod, R. and Hamilton, W. D. (1981). Science 211(4489), 1390-1396.