Short-Run vs Long-Run Adjustment in Competition

The same demand increase can have two different price effects over time.

Compare Three Entry and Exit Cases

The constant-cost cases start in long-run competitive equilibrium with identical firms, free entry and exit, unchanged technology and input prices, and a lasting demand shift. The short run allows variable-input and output adjustment while fixed inputs and seller count remain fixed. It is not the instant before any output can respond. Long-run zero economic profit is the endpoint under these assumptions, not a claim about every firm at every time.

Trace a Lasting Demand Increase

In the stated short-run adjustment after demand rises, the number of firms is fixed but existing firms can change variable inputs and move along their MC curves. With upward-sloping market supply and an initial zero-profit equilibrium, price, output per firm and economic profit rise. Entry subsequently expands market supply. In the constant-cost model, unchanged minimum ATC restores the original long-run price at a larger market output and seller count.

Read the Four-Stage Timeline

Build a timeline rather than placing all curves on one undifferentiated graph:

Separate a Loss, Shutdown and Exit

For a lasting demand decrease under the same assumptions, short-run price and output per firm fall. Firms may incur economic losses while continuing to operate if revenue covers variable cost; otherwise short-run shutdown can be preferable. Long-run exit reduces market supply until surviving identical firms again earn zero economic profit. Shutdown for a period and leaving the industry are different choices.

Identify Which Adjustment Stage Is Asked

When the question asks about the short run after variable inputs adjust but before entry, the stated model gives a higher price, more output per existing firm and positive economic profit. After full constant-cost adjustment, it gives the original price, zero economic profit, more firms and greater market output. A question about fixed inventory before production responds needs a different market-period assumption.

Check the Industry Cost Assumption

Constant-cost industry means expansion does not change firms’ input prices and cost curves; it does not mean an individual firm’s MC must be constant. With increasing industry costs, the new long-run zero-profit price is above the original benchmark under the usual assumptions. With decreasing industry costs, it can be below it. Do not promise that entry always reverses a particular fraction of the short-run price change.

Separate Market Supply from Firm MC

Entry adds sellers and shifts market supply in the constant-cost model. It does not by itself shift an incumbent’s unchanged MC curve. The incumbent moves along that curve as price changes. If input prices or technology change, its costs can shift too; then use the relevant cost-changing assumptions rather than the constant-cost shortcut.

Keep the Entry Incentive Visible

Temporary profit is the incentive for entry in this model. Setting short-run profit to zero simply because a long-run endpoint exists removes the very adjustment mechanism being analyzed. Keep the observed stage and the equilibrium being approached separate.

Distinguish Economic and Accounting Profit

Economic cost includes explicit payments such as wages, rent and materials plus relevant opportunity costs, including forgone owner earnings and the normal return on capital. Zero economic profit means revenue covers those economic costs. It does not require zero accounting profit or zero compensation for the owner.

In a separate bookkeeping example, $200 revenue minus $170 explicit cost gives $30 accounting profit. If opportunity cost is $30, economic profit is zero. The labels differ because the cost measures differ, not because the business has no income or because one profit number is an arithmetic error.

Know When the Benchmark Changes

For a cost-changing event, the new long-run endpoint may use a different minimum ATC. Returning to the old price requires the unchanged constant-cost assumptions. Persistent differences between firms, barriers to entry or scarce firm-specific resources also require more detail than the identical-firm zero-profit benchmark.

Use the Timeline Without Skipping a Stage

A disciplined timeline names three stages: the initial equilibrium, the short-run firm response with seller count fixed, and the long-run entry or exit response. If the stem stops at one stage, do not import conclusions from the next. This sequencing is more reliable than memorizing that competitive profit “always becomes zero” without asking when.

Read the Entry and Exit Explanation

OpenStax’s long-run entry and exit lesson explains the technical time horizons, short-run losses, free-entry adjustment and different industry cost conditions. The equations and questions below are original examples, not copied textbook exercises.

Calculate a Constant-Cost Adjustment

Each identical firm has total economic cost TC = q² + $100 and MC = 2q. The $100 is unavoidable over the short-run operating period but avoidable when deciding whether to enter or exit. Costs and technology stay unchanged as the industry expands. Initial demand is Q = 200 − 5P, where Q is industry output and q is one firm’s output. Ten firms initially operate.

ATC is q + 100/q for positive q. It is at least $20 because ATC − 20 = (q − 10)²/q, with equality at q = 10. Thus P = $20 supports the initial long-run equilibrium: each firm produces 10 units, the industry produces 100 and each firm earns zero economic profit. Demand then shifts permanently to Q = 300 − 5P.

Before entry, each firm chooses q = P/2, so ten firms supply Q = 5P. Set 5P = 300 − 5P to get P = $30, Q = 150 and q = 15. Revenue per firm is $450 and total cost is $325, leaving $125 economic profit. With free entry and unchanged costs, price returns to $20; demand is then 200, supplied by twenty firms producing ten units each. Firm output returns to ten while industry output doubles from its initial level.

Test a Separate Demand Decrease

Return to the original ten-firm equilibrium, then instead reduce demand to Q = 150 − 5P. What happens before exit, and why might a loss-making firm keep operating during the short run?

Try Three Adjustment Questions

Connect Entry, Barriers and Market Structure

Contrast free entry with obstacles that can protect a seller’s position, compare monopoly output choices under changing demand, or continue with the CLEP Microeconomics learning hub.