Source: Sample Midterm 1, Q5a and Q5b
Difficulty: Intermediate to Advanced | Prerequisites: Marginal utility, budget constraints, supply and demand basics, production possibilities curve (PPC).
This topic ties consumer theory to market outcomes and the broader economy. It starts at the individual level (how a utility-maximising household responds to a change in tastes) and scales up to market demand curves and the production possibilities curve. The question tests whether you can trace a change from micro-level consumer optimisation through to economy-wide reallocation. It combines several tools you have learned separately.
When tastes shift away from travel services, the marginal utility curve for travel shifts down. Households rebalance, consuming less travel and more of everything else, which shifts the household demand curve for travel leftward. Aggregating across households, the market demand curve shifts left, lowering both price and quantity. On the PPC, the economy moves from a point with more travel to one with less travel and more of everything else.
Marginal utility (MU)
The additional satisfaction from consuming one more unit of a good. It typically diminishes as you consume more.
Rational spending rule
A utility-maximising consumer allocates spending so that the marginal utility per pound (or dollar) is equal across all goods: MU_T / P_T = MU_E / P_E. In simple terms, the last pound spent on each good should give you the same bang for your buck.
Budget constraint
The set of all combinations of goods a consumer can afford given their income and prices. The consumer must choose a point on or inside the budget line.
Household demand curve
The curve showing the quantity of a good a household wants to buy at each price, holding other things constant. A change in tastes shifts it.
Market demand curve
The horizontal sum of all individual households' demand curves.
Production possibilities curve (PPC)
The curve showing all combinations of two goods an economy can produce using all its resources efficiently. It is bowed outward, reflecting increasing opportunity costs.
Step 1: The rational spending rule before the change
Before the taste change, the consumer chooses quantities of travel (T) and everything else (E) so that MU_T / P_T = MU_E / P_E.
Step 2: The taste change shifts the MU curve
A decline in the taste for travel shifts the MU curve for travel downward (from MU_T1 to MU_T2).
At the original quantities and prices, the consumer now has MU_T / P_T < MU_E / P_E. The last pound spent on travel gives less satisfaction than the last pound spent on everything else.
Step 3: The consumer rebalances
The household buys less travel (which pushes MU_T back up along the new, lower curve) and more of everything else (which pushes MU_E down) until MU_T / P_T = MU_E / P_E again.
The consumer also must stay on the budget constraint.
Step 4: The household demand curve shifts
At every price, the household now wants less travel. The household demand curve for travel shifts leftward (d₁ to d₂).
Market demand shifts
The market demand curve is the horizontal sum of all households' demand curves. If the typical household's demand has shifted left, so does the market demand curve (D₁ to D₂).
In the supply-and-demand diagram for travel services: equilibrium price falls and equilibrium quantity falls.
Movement along the PPC
The PPC for the economy shows the trade-off between travel services and everything else, with the usual bowed-out shape.
The change in tastes does not shift the PPC (no change in resources or technology). Instead, it causes a movement along the PPC, from a point like A (more travel) to a point like B (less travel, more of everything else).
It is essential to say this is a movement along the PPC, not a shift of it.
Rational spending rule:
\frac{MU_T}{P_T} = \frac{MU_E}{P_E}Diagrams needed:
Marginal utility diagrams for travel and everything else (showing the shift in MU_T and the resulting movement to new consumption points).
Household demand curve for travel (leftward shift from d₁ to d₂).
Market supply and demand for travel (S₁ stays, D shifts left from D₁ to D₂, price and quantity both fall).
PPC diagram (bowed out; movement from point A to point B along the curve).
During and after the pandemic, tastes shifted away from travel services and toward home goods. The rational spending rule tells you why: with lower marginal utility from travel, households reallocated their budgets. Market demand for travel fell, prices dropped, airlines cut capacity, and the economy's production mix shifted along its PPC toward more home goods and less travel.
Students sometimes skip the rational spending rule and jump straight to "demand shifts left." Without the rule, you cannot get full marks on part (a).
Confusing a shift of the PPC with a movement along it. A taste change does not change resources or technology, so the PPC stays put. The economy moves to a different point on it.
Just asserting that the market demand shifts without explaining the link to individual household demand curves. You should reason through the horizontal summation.
Forgetting the budget constraint. Even though the MU diagrams do most of the work, acknowledging that the consumer must stay on the budget line is important for a top answer.
⚠️ The rational spending rule is essential for part (a). An answer without it earns no more than 7 out of 15.
⚠️ At least one marginal utility diagram is expected for a top score on part (a).
⚠️ Mentioning the budget constraint is not required for full credit, but a perfect answer will reference it.
⚠️ For part (b), saying there is a movement along the PPC (not a shift) is critical. Missing this caps the answer at about 10 out of 15.
⚠️ If you just assert the market demand shifts without linking back to the household demand shift, expect a cap of about 13 out of 15.
Fill in the blank: The rational spending rule states MU_T / P_T = ______. (MU_E / P_E)
True or false: A decline in the taste for travel shifts the MU curve for travel upward. (False, it shifts it downward.)
True or false: A taste change shifts the PPC. (False, it causes a movement along the PPC.)
Fill in the blank: The market demand curve is the ______ sum of household demand curves. (horizontal)
True or false: After a taste shift away from travel, both equilibrium price and quantity of travel fall. (True.)
Q: Using the rational spending rule and a marginal utility diagram, explain how a permanent decline in the taste for travel services affects a utility-maximising household's consumption.
A: Before the taste change, the household sets MU_T / P_T = MU_E / P_E. The taste change shifts the MU_T curve downward. At the initial quantities, MU_T / P_T < MU_E / P_E, so the household is getting less satisfaction per pound from travel than from everything else. To restore equality, it buys less travel (moving up the new MU_T curve) and more of everything else (moving down the MU_E curve), subject to the budget constraint. The household demand curve for travel shifts left.
Q: Show how this taste change affects the market for travel services and the economy's position on the PPC.
A: The market demand curve for travel is the horizontal sum of household demand curves, so it shifts left (D₁ to D₂). With supply unchanged, the equilibrium price and quantity of travel both fall. On the PPC (travel vs everything else), the economy moves from a point with more travel (A) to one with less travel and more of everything else (B). This is a movement along the PPC, not a shift, because resources and technology have not changed.
This links to the supply-and-demand analysis of any market: the demand curve is ultimately grounded in consumer utility maximisation. It also connects to comparative advantage and the PPC, which shows how the economy's production mix responds to changes in preferences, prices, or trade opportunities.
Rational spending rule, marginal utility, MU per dollar, MU per pound, budget constraint, consumer choice, utility maximisation, taste change, demand shift, household demand curve, market demand curve, horizontal summation, production possibilities curve, PPC, PPF, movement along PPC, travel services, ECON 2, intro microeconomics, NYU Economics 2