You can fine the cheater by taking away some money, as long as you're willing to give up the same amount yourself. In other words, you can punish a cheater if you're willing to pay for the opportunity.
http://en.wikipedia.org/wiki/Wason_selection_task#Policing_s...
This experimental evidence supports the hypothesis that a Wason task proves to be easier if the rule to be tested is one of social exchange (in order to receive benefit X you need to fulfill condition Y) and the subject is asked to police the rule, but is more difficult otherwise. Such a distinction, if empirically borne out, would support the contention of evolutionary psychologists that certain features of human psychology may be mechanisms that have evolved, through natural selection, to solve specific problems of social interaction, rather than expressions of general intelligence. In this case, the module is described as a specialized cheater-detection module.
... groups with four members played the following public goods game. Each member received an endowment of 20 money units (MUs) and each one could contribute between 0 and 20 MUs to a group project. Subjects could keep the money that they did not contribute to the project. For every MU invested in the project, each of the four group members, that is, also those who invested little or nothing, earned 0.4 MUs. Thus, the investor's return from investing one additional MU in the project was 0.4 MUs, whereas the group return was 1.6 MUs.
Your return on an investment of 1U is 0.4U, so it's always best for you personally to keep a unit. But the group return is 1.6U so the group as a whole is enriched for every unit invested.
Each player of the pair begins with a set amount of money, say $5. Each puts any part or all of that $5 into a mutual pot, without knowing how much the other player is investing. Then a dollar is added to the pot, and the sum is split evenly between the two. So if both put in $5, they each wind up with $5.50 ($5 $5 $1, divided by 2). But suppose the first player puts in $5 and the second holds back, putting in only $4? The first player gets $5 at the end ($5 $4 $1, divided by 2), while the cheater gets $6 ($5 $4 $1, divided by 2--plus that $1 that was held back).
It seems to me that there isn't actually anything to be gained from cooperation. If both players "cheat" completely (put $0 into the pool), they still get $5.50. In that sense, the Nash equilibrium (both are cheating) is also socially optimal. Kind of untypical for something where you want to demonstrate the advantages of cooperation.