There are two efficiency effects of price controls: an “output effect” measured by the standard welfare loss triangles, and an “imperfect selection effect” that arises when controls prevent price from excluding high-cost sellers or low-value buyers. Although not discussed in most textbooks, the imperfect selection effect can be as large as the standard Harberger triangle welfare loss in symmetric designs, as confirmed by a class experiment described in this paper. The experiment also permits an analysis of the ways random non-price allocations shift the relevant supply function, and the related effects of rent-seeking competition that can arise with price controls.
