Question:
Consider the markets for fresh vegetables and instant noodles. Assume that fresh vegetables are a normal good, while instant noodles are an inferior good. Suppose Congress bans a commonly used fertilizer and pest-control chemical in vegetable farming. Without this input, vegetable yields fall due to increased spoilage and pest damage.
(a) Using a supply and demand diagram, explain how this policy affects the equilibrium price and quantity of fresh vegetables.
(b) Explain how the higher price of vegetables affects real household purchasing power.
(c) Taking into account that vegetables are a normal good and instant noodles are an inferior good, explain how the policy affects the demand for each good.
(d) Using a supply and demand diagram, show the resulting change in the equilibrium price and quantity of instant noodles.
(e) What is the unintended consequence of this regulation for people’s diets?
Solution:
Two features of this setup drive everything that follows: the normal/inferior distinction between the two goods, and the fact that the regulation raises the cost of producing vegetables. Together they determine how the policy affects prices, quantities, and household diets.
(a) The market for fresh vegetables
The ban does not initially make consumers want fewer vegetables. It makes vegetables more costly to supply. Without the banned input, farmers get fewer usable vegetables from a given amount of land, labor, and capital: some output is lost to lower yields, and some to greater pest damage. At any given price, farmers are willing and able to sell fewer vegetables than before.
In a standard supply and demand framework, this is a leftward shift of the vegetable supply curve. Demand has not shifted in the first instance. The market adjusts through a higher equilibrium price and a lower equilibrium quantity. Consumers pay more for vegetables and buy fewer of them.
(b) Purchasing power
The higher price of vegetables reduces real household income. Nominal income is unchanged, but a budget that previously bought some bundle of goods now buys less, because one of those goods costs more. A household that wants its old quantity of vegetables must spend more to get it, leaving less for everything else; a household that holds vegetable spending fixed must accept fewer vegetables. Either way the budget constraint tightens.
The size of this effect depends on how large vegetables loom in the household’s budget. For most households the share is modest, so the real-income loss from this one price increase is real but small. It matters here not because it is large, but because it is the channel through which a vegetable-market regulation reaches another food market.
(c) Demand for each good
It helps to separate two distinct effects, because they act differently on the two goods.
The substitution effect comes from the change in relative prices. Vegetables are now more expensive relative to instant noodles, so at the margin consumers move away from vegetables and toward noodles, holding real income constant.
The income effect comes from the loss of real purchasing power. Its direction depends on whether a good is normal or inferior. Vegetables are normal, so lower real income pushes vegetable consumption down. Noodles are inferior, so lower real income pushes noodle consumption up.
For vegetables, the substitution and income effects reinforce each other in reducing consumption. The higher relative price of vegetables induces consumers to move along the vegetable demand curve, buying fewer vegetables. The reduction in real income also shifts the demand for vegetables leftward, because vegetables are a normal good.
For noodles the two effects point the same way. Substitution raises noodle demand because vegetables become relatively more expensive; the income effect also raises noodle demand, because noodles are inferior and real income falls. This is the analytically interesting case: it is precisely because noodles are inferior that the income effect amplifies rather than offsets the substitution effect.
(d) The market for instant noodles
The regulation applies to vegetable farming, so the supply of noodles does not shift. What changes, in the first instance, is demand. From part (c), both the substitution effect and the income effect raise the demand for noodles, so the noodle demand curve shifts right. Along the unchanged noodle supply curve, this produces a higher equilibrium price and a higher equilibrium quantity. Consumers buy more noodles and pay more for them.
There is a feedback effect worth noting. As the noodle price rises, noodles become relatively less attractive than they were immediately after demand shifted. To the extent the two goods are substitutes, this higher noodle price raises the demand for vegetables relative to what it otherwise would have been, partially offsetting the leftward pressure on vegetable demand from part (c). This dampens the adjustment but does not reverse it: the original supply shock to vegetables is still there, so vegetables remain more expensive and less consumed than before the regulation, absent some reason to think the feedback is strong enough to overwhelm the initial shock.
(e) The unintended consequence
The unintended consequence follows directly from the price theory. A regulation aimed at restricting a chemical used in vegetable farming raises the cost of producing vegetables. Higher production costs reduce supply, raise the price, and lower the quantity consumed. Because households face limited budgets, the higher price also reduces real purchasing power, and some consumers substitute toward cheaper inferior goods, including instant noodles.
So a policy aimed at one margin can worsen outcomes on another. By making fresh vegetables more expensive, the regulation can lead people to eat fewer vegetables and more processed, less nutritious substitutes than they otherwise would have. The mechanism runs through constraints, relative prices, and the margins on which households actually adjust, not through anyone’s intentions.
