Price Controls and Antitrust in Imperfect Markets

A monopolist picks quantity where marginal revenue equals marginal cost, then charges the high price the demand curve allows at that quantity — always above marginal cost, and below the competitive output. Set a price ceiling exactly at marginal cost, below the monopoly's chosen price, and something strange happens: up to that ceiling quantity, the monopolist can't charge more than the ceiling no matter how much it produces, so effective marginal revenue becomes flat and equal to the ceiling price. It now pays to keep producing until marginal cost rises to meet that flat line, which happens at a higher quantity than before. Price falls and quantity rises together. This only works for ceilings placed between the monopoly price and the competitive price; push it below marginal cost and you get the ordinary shortage.

Antitrust policy attacks the same problem from the structure side instead of the price side. Rather than regulating the price a dominant firm charges, it tries to prevent or break up the concentrated market power itself — blocking mergers that would let firms raise price by restricting output, or breaking up existing monopolies — so competition does the disciplining instead of a regulator.

The same logic flips for a monopsony, a single buyer facing an upward-sloping labor supply curve, whose marginal cost of hiring rises faster than the wage. A monopsony hires too few workers at too low a wage. A wage floor set at the competitive wage removes the incentive to hold wages down to raise marginal cost, so hiring rises toward the competitive, efficient level — the mirror image of the price-ceiling result above.