Before revision:  Through simulating the evolution of Prisoners' dilemma game with three different hierarchical distributions: uniform, exponential and power-law , we find that the levels of cooperation are enhanced in all cases, and the enhancement is optimal in the uniform case.
After revision:  Through simulating the evolution of Prisoners' dilemma game with three hierarchical distributions , we find that the levels of cooperation are enhanced in all cases, and the enhancement is optimal in the uniform case.