Long-Run Equilibrium and Zero Economic Profit

Economic profit is a signal, and signals get arbitraged away. If firms in a competitive industry are earning positive economic profit in the short run, that profit is a flashing sign to outsiders: come make this too. New firms enter, which shifts market supply rightward. As supply grows, market price falls. Each existing firm, a price taker, sees its horizontal demand curve slide down with it. Entry keeps happening, and price keeps falling, until price lands exactly on the minimum point of the average total cost curve. There, price equals ATC, economic profit is zero, and the incentive to enter disappears. If firms were instead losing money, the same logic runs in reverse: losses push firms to exit, supply shifts left, price rises back to minimum ATC.

Picture the two graphs side by side. On the market graph, supply shifts right (or left) while demand holds still, sliding the equilibrium price down (or up) toward minimum ATC. On the firm graph, the firm's horizontal price line drops (or rises) until it just touches the bottom of the U-shaped ATC curve, at the same quantity where marginal revenue equals marginal cost. That tangency is the whole story in one picture: P equals MR equals MC equals minimum ATC.

Zero economic profit does not mean zero accounting profit or a dying industry. Economic profit already subtracts the opportunity cost of everything the owner contributes, so zero economic profit means the firm earns exactly as much as its next best alternative — a normal profit. That is a stable resting point, not a death spiral. This topic closes out the perfect competition model that the unit built from production functions and cost curves, and it sets up the contrast coming next with monopoly and other market structures, where blocked entry lets profit persist.