Your town has nineteen restaurants. Each is a little different — location, menu, atmosphere — so each can charge a bit more or less than rivals without losing every customer. That is monopolistic competition: many sellers, differentiated products (the monopolistic part), low barriers to entry (the competitive part), and heavy non-price competition (advertising, branding, quality).
Differentiation gives each firm its own downward-sloping but highly elastic demand curve. So the graph machinery is monopoly's (D with MR below it, two-step pricing), while the long-run logic is perfect competition's (entry erodes profit).
Short run: identical to monopoly analysis — Q where MR = MC, P from demand, profit or loss from P versus ATC.
Long run: profits attract entrants selling close substitutes → each incumbent's demand curve shifts left (and flattens) → profits shrink → entry stops when economic profit = 0. Graphically, demand ends up tangent to ATC exactly at the MR = MC output.
[GRAPH: Monopolistic competition, long run. Downward D tangent to U-shaped ATC at output q, with MR = MC directly below the tangency point. Price = ATC at q (zero economic profit). Minimum of ATC lies to the RIGHT of q*, gap labeled "excess capacity". P > MC gap visible.]
Long-run scorecard: - Zero economic profit (like perfect competition) - Not allocatively efficient: P > MC — some valued units go unproduced - Not productively efficient: output sits left of minimum ATC — the firm has excess capacity (it could produce more at lower average cost, but demand for its particular variety is too small) - These inefficiencies are the price society pays for variety
Conditions: a few large firms dominate, significant barriers to entry, products standardized (steel) or differentiated (wireless plans, airlines). The defining feature is mutual interdependence — each firm's best move depends on what rivals do. Two gas stations across an intersection watch each other's signs by the hour. Cost-curve graphs alone can't capture that, so we model oligopoly with game theory.
A payoff matrix shows each player's outcome for every strategy combination. Convention: the row player's payoff is listed first in each cell.
Two wireless carriers set plan prices High or Low. Payoffs (VoxTel, Nimbus) in millions:
| Nimbus: High | Nimbus: Low | |
|---|---|---|
| VoxTel: High | (10, 10) | (3, 14) |
| VoxTel: Low | (14, 3) | (6, 6) |
Reading protocol (do it mechanically): 1. VoxTel's best responses: if Nimbus plays High → compare 10 (High) vs. 14 (Low) → Low. If Nimbus plays Low → 3 vs. 6 → Low. Low wins either way → Low is VoxTel's dominant strategy. 2. Nimbus (symmetric): 10 vs. 14 → Low; 3 vs. 6 → Low → dominant strategy Low. 3. Nash equilibrium: a cell where neither player gains by unilaterally switching → (Low, Low) = (6, 6).
Definitions: - Dominant strategy: the best choice regardless of the rival's choice. (A player may have none.) - Nash equilibrium: a strategy pair where each player is doing their best given the other's choice. Check every cell: would either player deviate alone? A game can have one, several, or (in pure strategies) no Nash equilibrium. Dominant strategies are sufficient for Nash but not necessary — always run the cell-by-cell check.
This game is a prisoner's dilemma: both carriers prefer (10, 10), but individual incentives drag them to (6, 6). Self-interest defeats cooperation.
Collusion (illegal in the U.S.) or a cartel (OPEC is the classic example): firms agree to act like a shared monopoly — restrict output, raise price, split the profit. The payoff matrix explains both the appeal and the fragility: at the collusive cell (High, High), each firm can gain by secretly cheating (cut price, grab share: 10 → 14). Since both face that temptation, cartels tend to unravel toward the Nash equilibrium — especially with many members, hard-to-detect price cuts, or no way to punish cheaters. Repeated interaction softens the dilemma: future retaliation makes cheating costlier, which is why real oligopolies often sustain tacit cooperation (like price leadership) without any explicit agreement.
| Perfect competition | Monopolistic competition | Oligopoly | Monopoly | |
|---|---|---|---|---|
| Number of firms | Very many | Many | Few | One |
| Product | Identical | Differentiated | Either | Unique |
| Entry barriers | None | Low | High | Very high |
| Price control | None (taker) | Some | Substantial, interdependent | Most |
| Long-run economic profit | Zero | Zero | Possible | Possible |
| Efficiency (long run) | Allocative ✓ Productive ✓ | Neither (P > MC, excess capacity) | Generally neither | Neither |
| Example | Wheat farming | Local restaurants | Wireless carriers, two gas stations | Sole water utility |
1 — C. Correct: differentiation is the wedge — each firm's product is a little different, so each faces its own downward-sloping demand curve and has some price-setting power. A) entry barriers are low in monopolistic competition; that's the competitive half. B) easy entry drives long-run economic profit to zero. D) price-taking describes perfect competition; these firms are price makers. E) long-run output sits left of minimum ATC (excess capacity). Fix: hear "differentiated products + easy entry" and answer "downward-sloping demand now, zero profit later."
2 — A. Correct: entry by close substitutes shifts each incumbent's demand left until it is tangent to ATC at the MR = MC output — P = ATC, zero economic profit. B) differentiation tilts the demand curve but cannot stop entry from eroding profit. C) zero economic profit still means positive accounting profit — implicit costs are covered. D) losses trigger exit, which shifts remaining demand curves right, restoring zero profit — not perpetual losses. E) advertising is a cost inside ATC, not a benchmark that profit gravitates to. Fix: long-run monopolistic competition = tangency picture — D touches ATC exactly once, at the profit-maximizing quantity, and profit is zero.
3 — E. Correct: the tangency happens on ATC's downward slope, so the firm's output is smaller than the ATC-minimizing output — it has unused, "excess" capacity. A) reverses the direction of the gap. B) P > MC in monopolistic competition; P = MC never holds there. C) confuses a long-run structural property with business-cycle idleness. D) the allocatively efficient quantity requires P = MC, which these firms don't reach. Fix: tangency on the downslope of ATC → output short of minimum ATC → excess capacity, the graphical signature of monopolistic competition.
4 — B. Correct: each station's best price depends on the other's — instant matching is strategic reaction, the defining trait of oligopoly. A) identical prices can emerge in many structures; watching and matching a specific rival is not price-taking. C) both stations charge all drivers the same posted price — no segmentation. D) monopolistic competition has many rivals, so no single competitor's move matters this much. E) matching a posted price is independent best-responding; collusion requires an agreement, not just similar behavior. Fix: when the scenario stresses firms reacting to each other's moves, the answer is oligopoly/interdependence regardless of the product.
Questions 5–8 refer to this payoff matrix. Wireless carriers VoxTel and Nimbus each choose a High or Low price. Payoffs are (VoxTel, Nimbus) in millions:
| Nimbus: High | Nimbus: Low | |
|---|---|---|
| VoxTel: High | (10, 10) | (3, 14) |
| VoxTel: Low | (14, 3) | (6, 6) |
5 — D. Correct: check both columns — against High, Low pays 14 > 10; against Low, Low pays 6 > 3. Low wins in every case, the definition of a dominant strategy. A) joint-best outcomes don't define dominant strategies; unilateral best responses do. B) compares payoffs across the wrong cells (both-High vs. both-Low). C) a strategy that's best only sometimes is by definition not dominant. E) the two comparisons show Low dominates, so a dominant strategy exists. Fix: cover one rival column at a time and circle the row player's better payoff; a strategy circled in every column is dominant.
6 — C. Correct: at (Low, Low), VoxTel switching to High drops from 6 to 3 and Nimbus switching drops from 6 to 3 — no unilateral move helps, so it is Nash. A) (High, High) maximizes joint profit but either firm gains (10 → 14) by defecting, so it is not stable. B) and D) in each of those cells the low-payoff firm (earning 3) would switch and earn 6. E) a Nash equilibrium exists here — conflicting interests don't preclude one. Fix: test every cell with one question per player — "holding the rival fixed, can I do better by switching?" A cell with two 'no's is Nash.
7 — D. Correct: both firms play their individually best strategy (Low) and land at (6, 6), even though (10, 10) is better for each — the prisoner's dilemma. A) the price war ends at lower profits for both, not higher. B) dominant play produced the worst symmetric joint outcome here. C) oligopolists are price makers; the game is entirely about their price choices. E) the Nash outcome (6, 6) is precisely what both firms wish they could escape. Fix: never equate "Nash equilibrium" with "best outcome" — Nash means no unilateral improvement, not no better cell.
8 — B. Correct: from (High, High), either carrier that secretly cuts price jumps from 10 to 14 — and Low is dominant anyway — so each has a standing incentive to cheat on the pact. A) pricing choices don't change the cost of running a network. C) the matrix contains the whole market; outside exit isn't what unravels the deal. D) is numerically false — joint 20 at (High, High) beats joint 12 at (Low, Low); the problem is private temptation, not joint arithmetic. E) no such pricing regulation exists in the scenario. Fix: to explain cartel collapse, point to the cheater's payoff jump from the collusive cell — name the numbers (10 → 14).
9 — E. Correct: easy entry means profit invites new rivals whose close substitutes pull away customers — her demand shifts left (and flattens) until profit is zero. A) entry splits the market among more sellers; an individual incumbent's demand shrinks. B) barriers stay low by assumption; nothing about her profit builds them. C) entry works through her demand, not her cost curves. D) more competition pushes her price down, not up. Fix: in low-barrier markets, always translate "profit today" into "leftward demand shift for incumbents tomorrow, until profit is zero."
10 — A. Correct: both structures feature easy entry and exit, and entry grinds economic profit to zero in the long run. B) P = MC holds only in perfect competition; monopolistic competitors keep P > MC. C) minimum-ATC production is also exclusive to perfect competition — the other has excess capacity. D) differentiation is the defining trait of monopolistic competition, so products are not homogeneous. E) only perfect competitors face horizontal demand; differentiation tilts it. Fix: the two structures share exactly one long-run headline — zero economic profit; every efficiency property belongs to perfect competition alone.
11 — D. Correct: advertising is non-price competition — it differentiates the product, shifting the firm's demand curve rightward and making it less elastic, which supports a higher markup. A) advertising adds cost; it does not reduce the cost of cooking a meal. B) advertising deepens differentiation, moving the market away from perfect competition. C) rivals can advertise back and new firms can enter, so any profit gain erodes — nothing is permanent in a low-barrier market. E) advertising builds brand loyalty, which works like a mild barrier rather than removing one. Fix: file advertising under "demand-side strategy": its goal is to shift and steepen the firm's own demand curve, not to change costs.
12 — E. Correct: in the pricing game above, both firms choose Low independently — identical prices are exactly what noncooperative best-responding predicts, no agreement needed. A) identical prices are consistent with competition, tacit coordination, or collusion — they prove none of them. B) interdependent firms often converge on matching prices without any communication. C) oligopolists are price makers; that is why the game exists. D) explicit price-fixing agreements are illegal regardless of the direction prices move. Fix: to infer collusion you need evidence of an agreement; matching prices alone are equally the fingerprint of a Nash equilibrium.
1 — C. Correct: differentiation is the wedge — each firm's product is a little different, so each faces its own downward-sloping demand curve and has some price-setting power. A) entry barriers are low in monopolistic competition; that's the competitive half. B) easy entry drives long-run economic profit to zero. D) price-taking describes perfect competition; these firms are price makers. E) long-run output sits left of minimum ATC (excess capacity). Fix: hear "differentiated products + easy entry" and answer "downward-sloping demand now, zero profit later."
2 — A. Correct: entry by close substitutes shifts each incumbent's demand left until it is tangent to ATC at the MR = MC output — P = ATC, zero economic profit. B) differentiation tilts the demand curve but cannot stop entry from eroding profit. C) zero economic profit still means positive accounting profit — implicit costs are covered. D) losses trigger exit, which shifts remaining demand curves right, restoring zero profit — not perpetual losses. E) advertising is a cost inside ATC, not a benchmark that profit gravitates to. Fix: long-run monopolistic competition = tangency picture — D touches ATC exactly once, at the profit-maximizing quantity, and profit is zero.
3 — E. Correct: the tangency happens on ATC's downward slope, so the firm's output is smaller than the ATC-minimizing output — it has unused, "excess" capacity. A) reverses the direction of the gap. B) P > MC in monopolistic competition; P = MC never holds there. C) confuses a long-run structural property with business-cycle idleness. D) the allocatively efficient quantity requires P = MC, which these firms don't reach. Fix: tangency on the downslope of ATC → output short of minimum ATC → excess capacity, the graphical signature of monopolistic competition.
4 — B. Correct: each station's best price depends on the other's — instant matching is strategic reaction, the defining trait of oligopoly. A) identical prices can emerge in many structures; watching and matching a specific rival is not price-taking. C) both stations charge all drivers the same posted price — no segmentation. D) monopolistic competition has many rivals, so no single competitor's move matters this much. E) matching a posted price is independent best-responding; collusion requires an agreement, not just similar behavior. Fix: when the scenario stresses firms reacting to each other's moves, the answer is oligopoly/interdependence regardless of the product.
5 — D. Correct: check both columns — against High, Low pays 14 > 10; against Low, Low pays 6 > 3. Low wins in every case, the definition of a dominant strategy. A) joint-best outcomes don't define dominant strategies; unilateral best responses do. B) compares payoffs across the wrong cells (both-High vs. both-Low). C) a strategy that's best only sometimes is by definition not dominant. E) the two comparisons show Low dominates, so a dominant strategy exists. Fix: cover one rival column at a time and circle the row player's better payoff; a strategy circled in every column is dominant.
6 — C. Correct: at (Low, Low), VoxTel switching to High drops from 6 to 3 and Nimbus switching drops from 6 to 3 — no unilateral move helps, so it is Nash. A) (High, High) maximizes joint profit but either firm gains (10 → 14) by defecting, so it is not stable. B) and D) in each of those cells the low-payoff firm (earning 3) would switch and earn 6. E) a Nash equilibrium exists here — conflicting interests don't preclude one. Fix: test every cell with one question per player — "holding the rival fixed, can I do better by switching?" A cell with two 'no's is Nash.
7 — D. Correct: both firms play their individually best strategy (Low) and land at (6, 6), even though (10, 10) is better for each — the prisoner's dilemma. A) the price war ends at lower profits for both, not higher. B) dominant play produced the worst symmetric joint outcome here. C) oligopolists are price makers; the game is entirely about their price choices. E) the Nash outcome (6, 6) is precisely what both firms wish they could escape. Fix: never equate "Nash equilibrium" with "best outcome" — Nash means no unilateral improvement, not no better cell.
8 — B. Correct: from (High, High), either carrier that secretly cuts price jumps from 10 to 14 — and Low is dominant anyway — so each has a standing incentive to cheat on the pact. A) pricing choices don't change the cost of running a network. C) the matrix contains the whole market; outside exit isn't what unravels the deal. D) is numerically false — joint 20 at (High, High) beats joint 12 at (Low, Low); the problem is private temptation, not joint arithmetic. E) no such pricing regulation exists in the scenario. Fix: to explain cartel collapse, point to the cheater's payoff jump from the collusive cell — name the numbers (10 → 14).
9 — E. Correct: easy entry means profit invites new rivals whose close substitutes pull away customers — her demand shifts left (and flattens) until profit is zero. A) entry splits the market among more sellers; an individual incumbent's demand shrinks. B) barriers stay low by assumption; nothing about her profit builds them. C) entry works through her demand, not her cost curves. D) more competition pushes her price down, not up. Fix: in low-barrier markets, always translate "profit today" into "leftward demand shift for incumbents tomorrow, until profit is zero."
10 — A. Correct: both structures feature easy entry and exit, and entry grinds economic profit to zero in the long run. B) P = MC holds only in perfect competition; monopolistic competitors keep P > MC. C) minimum-ATC production is also exclusive to perfect competition — the other has excess capacity. D) differentiation is the defining trait of monopolistic competition, so products are not homogeneous. E) only perfect competitors face horizontal demand; differentiation tilts it. Fix: the two structures share exactly one long-run headline — zero economic profit; every efficiency property belongs to perfect competition alone.
11 — D. Correct: advertising is non-price competition — it differentiates the product, shifting the firm's demand curve rightward and making it less elastic, which supports a higher markup. A) advertising adds cost; it does not reduce the cost of cooking a meal. B) advertising deepens differentiation, moving the market away from perfect competition. C) rivals can advertise back and new firms can enter, so any profit gain erodes — nothing is permanent in a low-barrier market. E) advertising builds brand loyalty, which works like a mild barrier rather than removing one. Fix: file advertising under "demand-side strategy": its goal is to shift and steepen the firm's own demand curve, not to change costs.
12 — E. Correct: in the pricing game above, both firms choose Low independently — identical prices are exactly what noncooperative best-responding predicts, no agreement needed. A) identical prices are consistent with competition, tacit coordination, or collusion — they prove none of them. B) interdependent firms often converge on matching prices without any communication. C) oligopolists are price makers; that is why the game exists. D) explicit price-fixing agreements are illegal regardless of the direction prices move. Fix: to infer collusion you need evidence of an agreement; matching prices alone are equally the fingerprint of a Nash equilibrium.