For an industry that is subject to uniform yardstick regulation, we study cartel stability and the impact of cartels on the regulated price. In a theoretical model, an increase in the number of symmetric firms may facilitate collusion. Our laboratory experiment suggests that this effect is even stronger than what theory predicts. Theory predicts that firm-size heterogeneity hinders collusion, but leads to higher regulated prices if firms do not collude. In a laboratory experiment we find that the first effect is stronger, implying that in a more heterogeneous industry regulated prices are lower. (C) 2016 Elsevier B.V. All rights reserved.