help cooperators in the long run. Am. Nat. 181.451-463. Taylor. P.D.. Jon ker. L. 1978. Evolutionarily stable strategies and game dynamics. Math. Biosa. 40. 145-156. Trivers. R.L. 1971. The evolution of reciprocal altruism. Quart. Rev. Biol. 46. 35-57. Tucker.A.. 1950.A tyro-person dilemma. In: Ra