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Monopolistic competition features many firms selling differentiated products, free entry and exit, and some degree of market power for each firm due to differentiation. In the short run, firms can earn positive economic profit; in the long run, entry drives profit to zero (as in perfect competition). The long-run equilibrium has each firm on the downward-sloping portion of its LRAC (excess capacity), meaning the industry is not at minimum efficient scale. This is the cost of product variety.
Contrast long-run equilibrium in perfect competition (P = min LRAC) with monopolistic competition (P = LRAC but above min LRAC, with excess capacity). Examples from retail and restaurants make the model concrete.
Monopolistic competition sits between the two market structures you already know. From perfect competition, it inherits two features: many firms and free entry and exit. From monopoly, it inherits one: each firm has some market power because it sells a product that is distinct from its competitors' products. Think of restaurants, clothing brands, or hair salons. There are many of them, anyone can open one, but your preferred Thai restaurant is not a perfect substitute for the Vietnamese place next door. This product differentiation gives each seller a downward-sloping demand curve — if it raises its price slightly, it loses some but not all customers. This is the defining characteristic that separates monopolistic competition from perfect competition, where the firm faces a horizontal demand curve.
In the short run, monopolistically competitive firms behave like monopolists within their niche: they set output where MR = MC and charge a price above marginal cost. If demand is strong enough, they earn positive economic profit. But this profit is temporary. Free entry means new competitors will enter, stealing customers and shifting each incumbent's demand curve leftward and making it more elastic. This entry continues until economic profit is driven to zero — the same long-run result as perfect competition. The mechanism is identical: profit attracts entry, entry reduces demand for each existing firm, profits fall until the incentive to enter disappears.
The long-run equilibrium looks different from perfect competition's, however. In perfect competition, zero-profit equilibrium occurs where P = min LRAC — the firm operates at efficient scale, the lowest possible average cost. In monopolistic competition, zero-profit equilibrium requires only that the demand curve be *tangent* to the LRAC curve — touching it at a point to the left of the minimum. At this tangency point, price equals average cost (zero profit), but the firm is operating with excess capacity: it is producing less than the output that would minimize average cost. Average cost is higher than the minimum achievable, and price exceeds marginal cost (P > MC), so the market outcome is not allocatively efficient.
This excess capacity theorem is the key welfare result for monopolistic competition. The gap between actual output and efficient-scale output is the cost of product variety. Society gets a wider range of differentiated products — more flavor options, more restaurant cuisines, more clothing styles — but pays for it in higher average costs and prices than a world of standardized goods would require. Whether this tradeoff is worthwhile is a normative question: consumers may value variety enough to accept higher costs. The point is simply that variety is not free, and monopolistic competition makes the price of variety explicit.