COOPERATION MAINTAINED BY FITNESS ADJUSTMENT.

QUESTIONS: Whether or not cooperation can be enhanced if players with a performance higher than the mean are forced to pay an additional cost in each generation? MATHEMATICAL METHODS: Analysis of replicator dynamics with mutation. The ESS distribution of cooperation level is obtained. KEY ASSUMPTIONS: Players engage in cooperative dilemma game, and at the end of each generation, those with higher performance than the mean are forced to pay additional cost. CONCLUSIONS: Without mutation, the entire population eventually conforms to a single cooperation level determined by the initial composition of the population. With mutation, there is an equilibrium distribution of cooperation level, which has a peak at an intermediate level of cooperation. Whether it is institutionalized such as tax or just a social custom, fitness adjustment based ultimately on people's emtion of "envy" is able to maintain cooperation.