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Interactive visualization of the short-run to long-run adjustment under monopoly and monopolistic competition. Watch entry and exit shift the demand curve in monopolistic competition until P=ATC, while monopoly maintains profit behind barriers to entry. Apply MR=MC to find Q*, track the profit rectangle, and identify the LR tangency and excess capacity.
Market Structure
Initial Scenario
Once Q* is found, the firm reads P* off the demand curve at Q*:
P* = a − b × Q* (where a is demand intercept, b is slope)
Economic profit per period:
π = (P* − ATC*) × Q*
Positive when P* > ATC* — green rectangle
Zero when P* = ATC* — break-even
Negative when P* < ATC* — red rectangle (loss)
Barriers to entry prevent new competition:
Demand curve is stable — no leftward shift from entry
If π > 0 in SR, it persists in LR (no adjustment mechanism)
If π < 0 in SR, firm considers shutdown vs. continue at a loss
Shutdown rule: continue if P ≥ AVC (covers variable costs)
No LR tangency condition — ATC does not have to touch demand
Free entry and exit act as the equilibrating force:
SR profit (P > ATC) → new firms enter → individual demand shifts LEFT → demand becomes more elastic
SR loss (P < ATC) → firms exit → remaining demand shifts RIGHT
Adjustment continues until:
P = ATC (zero economic profit)
Demand curve is exactly tangent to ATC
MR = MC still holds at the same Q*
At LR equilibrium for monopolistic competition:
The slope of demand = slope of ATC (tangency, not intersection)
This guarantees P = ATC and MR = MC simultaneously
Q_LR < Q at minimum ATC → excess capacity exists
Q_min = sqrt(Fixed Costs / atc_slope)
LR Q* for MC is always below Q_min
Excess capacity = Q_min − Q*_LR
Society pays a higher price for variety (product differentiation)
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Avoid these frequent errors
COMMON MISTAKES IN MARKET STRUCTURE ANALYSIS
Reading price from MR=MC directly: MR=MC gives Q*. Price is always found from the demand curve at Q*. Writing P = MC (or P = MR) is the profit-maximization error.
Believing monopolistic competition reaches minimum ATC in LR: The LR equilibrium requires demand to be tangent to ATC, which occurs on the downward-sloping portion of ATC — always to the left of the minimum. Excess capacity is a structural feature, not an accident.
Assuming zero profit means P = MC: In monopolistic competition, P = ATC in LR (zero economic profit), but P > MC (markup still exists). These are two different conditions.
Ignoring the difference between shutdown and exit: In the short run, a firm shuts down if P < AVC. Exit from the industry is a long-run decision when economic profit is persistently negative.
Treating the MR curve as linear when demand is non-linear: For linear demand P = a − bQ, MR = a − 2bQ. The MR curve has the same intercept but twice the slope — a common calculation error.
Applying monopolistic competition LR condition to monopoly: The P = ATC tangency is ONLY the LR equilibrium condition for free-entry markets. Monopoly with barriers to entry has no equivalent automatic adjustment.
Strategic insights for success
EXAM TIPS FOR MARKET STRUCTURE QUESTIONS
Always identify the market structure first: the number of firms, product differentiation, and barriers to entry determine which framework applies.
For any profit-maximization question, start with MR = MC → Q* → read P* from demand → compare P* to ATC* for profit status.
In monopolistic competition, the LR equilibrium has three simultaneous conditions:
1. MR = MC (profit maximization)
2. P = ATC (zero economic profit from entry)
3. Demand is tangent to ATC (graphical condition)
Excess capacity is always present in LR monopolistic competition. If an exam question asks about productive efficiency, the answer is: monopolistic competition is NOT productively efficient.
For monopoly questions on LR profit: profit persists if and only if barriers to entry persist. Regulatory, patent, or natural barriers are the typical CFA contexts.
On diagrams, remember:
MR curve starts at the same y-intercept as demand but falls at twice the rate.
ATC is U-shaped; MC passes through its minimum from below.
The markup P − MC is positive in both monopoly and monopolistic competition — this distinguishes both from perfect competition where P = MC.
Deadweight loss question: monopoly creates deadweight loss (P > MC, Q < competitive Q). Monopolistic competition creates less DWL due to product differentiation benefits, but is still not allocatively efficient.