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The distribution of the global externality, not its size, affects cooperation in the population. In the left panels, γ = ± 1; in the right panels, = ± 2.5. Comparing the bottom row with the top row shows that cooperation is more strongly favoured when group selection is stronger relative to individual-level selection. Each data point summarizes the mean over 2500 time steps from 25 simulations conducted for the same initial conditions. (Online version in colour.)
