Monopolistic Competition

Because each seller's product is distinguishable, a price increase loses some customers but not all of them. That gives every firm its own downward-sloping demand curve, and with it a marginal revenue curve that lies below demand. The profit-maximizing decision is the standard one: produce the quantity at which marginal revenue equals marginal cost, then read the price off the firm's own demand curve at that quantity. Price therefore exceeds marginal cost, and the gap widens as demand becomes less elastic.

In the short run the outcome depends on where average total cost sits relative to that price. If price exceeds average total cost the firm earns economic profit; if it lies between average variable cost and average total cost the firm loses money but continues operating to cover part of its fixed costs; below average variable cost, shutting down is the better choice.

Low barriers mean that visible economic profit attracts imitators. New entrants launch close substitutes, which pulls each incumbent's demand curve leftward and makes it flatter, because customers now have more alternatives. Entry continues until the typical firm's demand curve is just tangent to its average total cost curve. At that point price equals average total cost and economic profit is zero, though the firm still earns a normal return on capital. Losses trigger the reverse process: exit, remaining firms' demand shifting outward, and a return to break-even.

The tangency occurs on the downward-sloping portion of average total cost, not at its minimum. Each firm therefore operates with excess capacity, producing less than the output that would minimize unit cost, and price remains above marginal cost. Neither productive nor allocative efficiency is achieved, but consumers receive product variety in exchange.

When technology and cost curves are broadly similar across firms, competition migrates from price to perceived difference: formulation, design, packaging, service, distribution reach and brand association. Advertising is not incidental spending in this structure; it is the mechanism through which a firm steepens its own demand curve, softens elasticity and defends its markup. Sceptics call it a deadweight cost that raises average total cost; the counter-argument is that it conveys information, signals quality commitment and disciplines firms that would otherwise let quality slip.

The fingerprints appear directly in reported financials. Consumer-staples, restaurant, apparel, personal-care and asset-management businesses typically show heavy selling, general and administrative expense relative to revenue, healthy gross margins alongside thin operating margins, and balance sheets carrying acquired brand intangibles. Persistent high returns usually indicate something beyond differentiation, such as scale, regulation or distribution control.

Expect to classify a described industry by counting sellers, judging substitutability, testing barriers to entry and asking whether firms react strategically to one another. Then map the classification onto predictions: markup above marginal cost, an elastic but not perfectly elastic demand curve, non-price competition, and zero economic profit once entry has run its course. You should be able to describe the markup relationship qualitatively, that the proportional gap between price and marginal cost is the inverse of the absolute price elasticity of demand, so more elastic demand forces a thinner markup.

Be ready to link the structure to concentration measures and their limits. A low N-firm concentration ratio or Herfindahl-Hirschman Index is consistent with monopolistic competition, but neither captures entry conditions or imports, so both can misclassify an industry that is contestable or globally competitive.