The longer (i.e., more iterations) you spend in the shaded triangles of defection the more you’ll be pulled to the defect-defect equilibrium as a natural reaction to what the other person is doing and the outcome you’re getting. The longer you spend in the middle “wedge of cooperation”, the more you’ll end moving up and to the right in Pareto improvements. So we want to make that wedge bigger.
The size of that wedge is determined by the ratio of a player’s outcome from C-C to their outcome in D-D. In this case the ratio is 2:1, so the wedge is between the slopes of 2 and 1⁄2. If C-C only guaranteed 1.1-1.1 to each player while a defection got them at least 1, the wedge would be a tiny sliver. Conversely, if the payoff for C-C was 999-999 almost the entire square would be the wedge.
But the bigger the wedge, the more difference there is between outcomes on the pareto frontier so the outcome of 100% C-C is a lot less stable than if any deviation from it immediately led to non-equilibrium points that degenerate to D-D.
The longer (i.e., more iterations) you spend in the shaded triangles of defection the more you’ll be pulled to the defect-defect equilibrium as a natural reaction to what the other person is doing and the outcome you’re getting. The longer you spend in the middle “wedge of cooperation”, the more you’ll end moving up and to the right in Pareto improvements. So we want to make that wedge bigger.
The size of that wedge is determined by the ratio of a player’s outcome from C-C to their outcome in D-D. In this case the ratio is 2:1, so the wedge is between the slopes of 2 and 1⁄2. If C-C only guaranteed 1.1-1.1 to each player while a defection got them at least 1, the wedge would be a tiny sliver. Conversely, if the payoff for C-C was 999-999 almost the entire square would be the wedge.
But the bigger the wedge, the more difference there is between outcomes on the pareto frontier so the outcome of 100% C-C is a lot less stable than if any deviation from it immediately led to non-equilibrium points that degenerate to D-D.