  A principal provides nondiscriminatory incentives for independent and
identical agents. The principal cannot observe the agents' actions, nor does
she know the entire set of actions available to them. It is shown, very
generally, that any worst-case optimal contract is nonaffine in performances.
In addition, each agent's pay must depend on the performance of another. In the
case of two agents and binary output, existence of a worst-case optimal
contract is established and it is proven that any such contract exhibits joint
performance evaluation -- each agent's pay is strictly increasing in the
performance of the other. The analysis identifies a fundamentally new channel
leading to the optimality of nonlinear team-based incentive pay.
