Placing production units under common ownership is often suggested as a solution to the problem of externalities. This will not always be true when there are decreasing returns to scale. An atomistic industry could be more efficient than a monopoly in some instances. Even when the "optimal" industry configuration would involve a finite number of producers, no two may have appropriate incentives to combine. An omniscient and benign regulator can always assure a more efficient outcome than would result from the combination of private producers. Whether real-world regulators should be called upon, however, is less clear.