  We consider the problem of detecting norm violations in open multi-agent
systems (MAS). We show how, using ideas from scrip systems, we can design
mechanisms where the agents comprising the MAS are incentivised to monitor the
actions of other agents for norm violations. The cost of providing the
incentives is not borne by the MAS and does not come from fines charged for
norm violations (fines may be impossible to levy in a system where agents are
free to leave and rejoin again under a different identity). Instead, monitoring
incentives come from (scrip) fees for accessing the services provided by the
MAS. In some cases, perfect monitoring (and hence enforcement) can be achieved:
no norms will be violated in equilibrium. In other cases, we show that, while
it is impossible to achieve perfect enforcement, we can get arbitrarily close;
we can make the probability of a norm violation in equilibrium arbitrarily
small. We show using simulations that our theoretical results hold for
multi-agent systems with as few as 1000 agents---the system rapidly converges
to the steady-state distribution of scrip tokens necessary to ensure monitoring
and then remains close to the steady state.
