  In repeated-game applications where both the collusive and non-collusive
outcomes can be supported as equilibria, researchers must resolve underlying
selection questions if theory will be used to understand counterfactual
policies. One guide to selection, based on clear theoretical underpinnings, has
shown promise in predicting when collusive outcomes will emerge in controlled
repeated-game experiments. In this paper we both expand upon and experimentally
test this model of selection, and its underlying mechanism: strategic
uncertainty. Adding an additional source of strategic uncertainty (the number
of players) to the more-standard payoff sources, we stress test the model. Our
results affirm the model as a tool for predicting when tacit collusion is
likely/unlikely to be successful. Extending the analysis, we corroborate the
mechanism of the model. When we remove strategic uncertainty through an
explicit coordination device, the model no longer predicts the selected
equilibrium.
