To answer the question in the title: Negative supply shocks—shifts to the left in the short-run aggregate supply (SRSAS) curve—and positive demand shocks—shifts to the right in the aggregate demand (AD) curve—both contributed to the acceleration in inflation that began in the spring of 2021. But were the aggregate supply shifts, such as the semiconductor shortage that reduced the supply of new automobiles, more or less important than the aggregate demand shifts, such as the expansionary monetary and fiscal policies?
Adam Hale Shapiro of the Federal Reserve Bank of San Francisco used a basic piece of microeconomic analysis to estimate the contribution of shifts in aggregate supply and shifts in aggregate demand to inflation during this period. He looked at the prices of the more than 100 categories of goods and services in the personal consumption expenditures(PCE) price index. The PCE price index is a measure of the price level similar to the GDP deflator, except it includes only the prices of goods and services from the consumption category of GDP. Changes in the PCE price index are the Federal Reserve’s preferred measure of the inflation rate because that index includes the prices of more goods and services than are included in the consumer price index (CPI).
Shapiro explains how he used microeconomic reasoning to determine whether prices in one of the more than 100 categories of goods and services were increasing because of shifts in supply or because of shifts in demand:
“Shifts in demand move both prices and quantities in the same direction along the upward-sloping supply curve, meaning prices rise as demand increases. Shifts in supply move prices and quantities in opposite directions along the downward-sloping demand curve, meaning prices rise when supplies decline.”
For example, the figure on the left shows the effect on the market for toys of an increase in the demand for toys. (We discuss how shifts in demand and supply curves in a market affect equilibrium price and quantity in Chapter 3, Section 3.4 of Economics, Macroeconomics, and Microeconomics.) The demand curve for toys shifts to the right from D1 to D2, the equilibrium price increases from P1 to P2, and the equilibrium quantity increases from Q1 to Q2. The figure on the right shows the effect on the market for toys if the price increase results from a decrease in the supply of toys rather than from an increase in demand. The supply curve shifts to the left from S1 to S2, the equilibrium price increases from P1 to P2, and the equilibrium quantity decreases from Q1 to Q2.
Shapiro used statistical methods to determine the part of a change in price or quantity that was unexpected. He took this approach in order to focus on short-run changes in these markets caused by shifts in demand and supply rather than long-run changes resulting from “factors such as technological improvements, cost-of-living adjustments to wages, or demographic changes like population aging.” In some cases, the quantity or the price in a market were very close to their expected values, so Shapiro labeled the cause of a price increase in this market as “ambiguous.”
Shapiro notes that: “Categories that experience frequent supply-driven price changes include food and household products such as dishes, linens, and household paper items. Categories that experience frequent demand-driven price changes include motor vehicle-related products, used cars, and electricity.”
The following figure shows Shapiro’s results for the period from January 2020 through April 2022. The height of each column gives the inflation rate in the month measured as the percentage change in the PCE price index from the same month in the previous year. For example, in March 2022, the inflation rate was 6.6 percent. The height of the yellow segment is the part of inflation in that month attributable to increases in demand, the height of the green segment is the part of the inflation in that month that is attributable to decreases in supply, and the height of the green segment is the part of the inflation that Shapiro can’t assign to either demand or supply. In March 2022, increased in demand accounted for 2.2 percentage points of the total 6.6 percentage point increase in inflation. Decreases in supply accounted for 3.3 percentage points, and the remaining 1.2 percentage points had an ambiguous cause.
We can conclude that, measured this way, the increase in inflation from the spring of 2021 through the spring of 2022 was due more to negative supply shocks than to positive demand shocks.
Source: Adam Hale Shapiro, “How Much Do Supply and Demand Drive Inflation?” Federal Reserve Bank of San Francisco Economic Letter, 22-15, June 21, 2022.