Prisoner's Dilemma strategies
Grudger (Grim Trigger)
Co-operate until the opponent defects once, then defect for ever.
Played by Holdfast Athletic Known in the literature as Grim Trigger
Where it comes from
Also known as Grim Trigger. The strictest possible enforcement rule, and a standard reference point for how much punishment is too much.
How it plays
Grudger is the most unforgiving strategy in the standard set and one of the most effective, which surprises people who expect the moral of Axelrod to be that forgiveness wins. It opens co-operatively, it never defects first, and it answers a single defection with permanent punishment. There is no route back.
Against honest opponents it is indistinguishable from Tit For Tat and collects the same high mutual pay-off, and against a committed defector it is fully protected from the second round onwards. That combination gets it a long way, and its threat has a real property that softer rules lack: it is completely credible, because it has never once been withdrawn.
The cost arrives with noise. If a defection can happen by accident, then Grudger will eventually punish an opponent who did not mean it, for ever, and forfeit every co-operative pay-off that relationship still had in it. In a clean game it is close to optimal enforcement. In a messy one it burns down relationships that were worth keeping.
On a pitch it is a side that plays you completely straight until the first cynical challenge, and then presses you relentlessly for the rest of the season. The threat works, mostly. What it cannot do is take a bad tackle as a bad tackle rather than a declaration.
In this league
Holdfast Athletic of Holdfast play Grudger. That patience and mercilessness are the same trait, applied in order. They have not played yet this season.