一直没看懂这段内容，而最近很多讨论合作与惩罚方面的文献都是以这个模型为基础，恳请哪位高手指点迷津！！本人在此十分感谢。要想看原文在下面，那样更方便于理解。
To see why, consider a model in which a large population is
divided into groups of size n. There are two behavioral types,
contributors and defectors. Contributors incur a cost c to
produce a total benefit b that is shared equally among group
members. Defectors incur no costs and produce no benefits. If
the fraction of contributors in the group is x, the expected payoff
for contributors is bx-c and the expected payoff for defectors
is bx, so the payoff disadvantage of the contributors is a constant
c independent of the distribution of types in the population. Now
add a third type, ‘‘punishers’’ who cooperate and then punish
each defector in their group, reducing each defector’s payoff by
p/n at a cost k/n to the punisher. If the frequency of punishers
is y, the expected payoffs become b(x+y)-c to contributors,
b(x+y)-py to defectors, and b(x+y)-c-k(1-x-y)to
punishers. Contributors have higher fitness than defectors if
punishers are sufficiently common that the cost of being pun-
ished exceeds the cost of cooperating (py>c). Punishers suffer
a fitness disadvantage of k(1-x-y) compared with nonpun-
ishing contributors. Thus, punishment is altruistic and mere
contributors are ‘‘second-order free riders.’’