β>cb
- βhow far an actor's cooperativeness predicts its partner's (set, and measured from the meetings)
- cthe fitness cost the actor pays to help
- bthe fitness benefit its partner receives
The rule on the slide is the assortment form of Hamilton's rule (Fletcher and Doebeli, 2009). In this toy model each dot meets partners every generation: with probability β the partner is its own kind, otherwise anyone. A cooperator pays c and its partner gains b, so a cooperator expects βb + (1 − β)pb − c and a defector (1 − β)pb, where p is the share of cooperators. The gap is βb − c, which is why cooperators spread exactly when β > c/b, whatever p is. Random drift, mutation and a finite population blur that edge.