Why Supply Shocks Matter Over the Business Cycle
A business-cycle change in output or prices need not begin with a change in spending. A sudden rise in the cost of an important input, for example, can make production less profitable at each price level. Firms may then supply less output, even if demand has not changed. A favorable change in productivity or input availability can work in the opposite direction. Tracing these shifts helps explain why output and the price level can move in different combinations over the cycle.
The central short-run result is that an adverse aggregate supply shock tends to reduce real output and raise the price level. This combination can mean recessionary pressure alongside rising prices. A favorable aggregate supply shock tends to raise output and lower the price level. These are model-based directional results: the size of the changes depends on the slopes of aggregate demand and short-run aggregate supply.
The Short-Run AD–AS Model
Aggregate demand (AD) describes combinations of the overall price level and real output demanded, holding the other influences on spending fixed. In the short-run aggregate supply (SRAS) model, a higher price level is associated with more output supplied, other things equal. For a transparent calculation, suppose both curves are linear:
An adverse shock can represent a rise in production costs or a disruption that limits the availability of inputs. A favorable shock can represent a fall in costs or an improvement that allows firms to produce more at a given price level. The model summarizes the effect on supply; it does not require every firm or industry to be affected identically.
The direction of the shock is important. An adverse shock shifts SRAS upward and to the left: at a given output, firms require a higher price level to supply that output. A favorable shock shifts SRAS downward and to the right: at a given output, firms are willing and able to supply at a lower price level.
Why an Adverse Supply Shock Raises Prices and Lowers Output
The equilibrium calculation makes the result precise. Set the two curves equal and solve for output. Substituting that output into either curve gives the equilibrium price level.
The asterisk marks an equilibrium value. To see why an adverse shift has the stated signs, compare equilibria before and after an increase in the SRAS intercept \(c\), while holding AD fixed. If the intercept rises by \(\Delta c>0\), then:
The output result follows because the higher SRAS intercept increases the gap \(c\) between the curves at any given output; equilibrium output must fall to restore equality. For price, use AD: when equilibrium output falls, \(P=a-bY\) rises because \(b>0\). The equations also show that the size of each change depends on the slopes, not just on the shock. With a favorable shock, \(\Delta c<0\), so the signs reverse: equilibrium output rises and the price level falls.
Worked Examples
Worked Example: Calculating an Adverse Cost Shock
Suppose an economy has AD \(P=140-2Y\) and SRAS \(P=60+2Y\). Both \(P\) and \(Y\) are index values. An input-cost shock shifts SRAS upward in parallel by 12 price-index points. Calculate the initial and new equilibria and describe the changes.
Method. Equate AD and SRAS at each equilibrium. The initial SRAS intercept is 60; after the adverse shock it is \(60+12=72\).
Initial equilibrium. Set the curves equal: \(140-2Y=60+2Y\). Thus \(80=4Y\), so \(Y=20\). Substituting into AD gives \(P=140-2(20)=100\). The initial equilibrium is \(Y=20,\ P=100\).
New equilibrium. The shifted SRAS curve is \(P=72+2Y\). Set it equal to AD: \(140-2Y=72+2Y\). Thus \(68=4Y\), so \(Y=17\). Substituting into AD gives \(P=140-2(17)=106\). The new equilibrium is \(Y=17,\ P=106\).
Check and interpretation. The new SRAS gives \(72+2(17)=106\), matching AD. Output falls by \(17-20=-3\) index units, and the price level rises by \(106-100=6\) index points. This is the adverse-supply pattern. The common mistake is to use the lower output in SRAS but overlook that the upward shift raises the price level; checking both curves confirms the result.
Worked Example: Tracing a Favorable Productivity Shock
Suppose AD is \(P=150-3Y\) and SRAS is initially \(P=54+3Y\). A productivity improvement shifts SRAS downward in parallel by 12 price-index points. Find both equilibria and interpret the direction of change.
Method. Solve AD equal to SRAS before and after the shift. A downward shift of 12 changes the intercept from 54 to \(54-12=42\).
Initial equilibrium. Solve \(150-3Y=54+3Y\). Then \(96=6Y\), so \(Y=16\). The price level is \(P=150-3(16)=102\). Thus the initial equilibrium is \(Y=16,\ P=102\).
New equilibrium. The new SRAS curve is \(P=42+3Y\). Solve \(150-3Y=42+3Y\), giving \(108=6Y\) and \(Y=18\). The price level is \(P=150-3(18)=96\).
Check and interpretation. At the new output, SRAS gives \(42+3(18)=96\), equal to AD. Output rises by 2 index units and the price level falls by 6 points. The common mistake is to treat a downward shift in SRAS as a movement along the original SRAS curve. The productivity change shifts the curve itself; the equilibrium then moves to a new intersection.
Worked Example: Inferring the Size of a Supply Shift
An economy has AD \(P=180-2Y\) and initial SRAS \(P=60+Y\). After a shock, the observed equilibrium is \(Y=34,\ P=112\), while AD is unchanged. Find the new SRAS intercept, the size and direction of the shift, and verify the observed equilibrium.
Method. Use the observed equilibrium point in the SRAS equation \(P=c+Y\) to infer \(c\), then compare it with the original intercept.
Infer the new intercept. Substitute \(P=112\) and \(Y=34\): \(112=c+34\), so \(c=78\). The new SRAS curve is \(P=78+Y\). Since the original intercept was 60, the intercept rose by \(78-60=18\) points. The shift is adverse.
Verify the intersection. At \(Y=34\), AD gives \(P=180-2(34)=180-68=112\). The new SRAS gives \(P=78+34=112\). Both curves pass through the observed equilibrium.
Interpretation. With unchanged AD, the adverse supply shift is consistent with a lower equilibrium output and a higher price level than before. The common mistake is to infer the shock from the price change alone. The observed output and price must satisfy both curves, and the inferred SRAS intercept must be compared with its original value.
Interpreting the Cycle and Avoiding Traps
If an adverse supply shock reduces output below its earlier level, the economy experiences a contractionary effect even though the price level rises. When the price level is increasing over a period, that increase contributes to inflation during that period. The model therefore allows recessionary output and upward price pressure to occur together; falling output does not guarantee that prices are falling.
Keep the price level distinct from the inflation rate. A higher equilibrium price level after a shock means prices are higher than before the shock. It does not, by itself, imply that the price level will keep rising in later periods. Inflation is a rate of change in the price level, so a one-time level increase and continuing inflation are not the same claim.
Likewise, the model’s result is a change relative to the initial equilibrium or a no-shock comparison. It does not establish that the economy is in recession in the formal dating sense: output could fall from a high starting point and remain above a benchmark, or output could already be weak before the shock. Use the output gap or other recession measures when the question asks about the economy’s position relative to trend or about recession dating.
- Do not reverse the signs. An adverse parallel SRAS shift raises the intercept; a favorable shift lowers it.
- Do not call every price increase demand-driven. An adverse supply shift can raise the price level while reducing output.
- Do not confuse a curve shift with movement along a curve. The shock changes the SRAS schedule; the new equilibrium is found at its intersection with AD.
- Do not infer exact magnitudes from direction alone. The slopes \(b\) and \(d\) determine how a given shift is divided between output and the price level.
Check Your Understanding
For each problem, solve for the requested equilibrium or shift and check that the reported point satisfies both curves.
- AD is \(P=126-2Y\), and initial SRAS is \(P=54+2Y\). A cost shock raises the SRAS intercept by 8. Find the initial and new equilibria. (Answer: Initially \(Y=18,\ P=90\); after the shock \(Y=16,\ P=94\).)
- AD is \(P=144-2Y\), and initial SRAS is \(P=48+2Y\). A favorable supply change lowers the SRAS intercept to 40. Find the new equilibrium and the changes from the initial equilibrium. (Answer: Initially \(Y=24,\ P=96\); afterward \(Y=26,\ P=92\), so output rises by 2 and the price level falls by 4 index units.)
- AD is \(P=170-Y\), and initial SRAS is \(P=50+Y\). An adverse shock raises the SRAS intercept by 20. Calculate both equilibria and the percentage changes in output and the price level, using the initial values as denominators. (Answer: Initially \(Y=60,\ P=110\); afterward \(Y=50,\ P=120\); output changes by \(-16.6667\%\) and the price level by \(+9.0909\%\).)
- AD is \(P=200-3Y\), and initial SRAS is \(P=80+Y\). After a supply shock, equilibrium is observed at \(Y=26,\ P=122\), with AD unchanged. Infer the new SRAS intercept and the size and direction of its shift. (Answer: The new intercept is 96, an adverse upward shift of 16 points.)
- Initially, AD is \(P=160-2Y\) and SRAS is \(P=40+2Y\). Then an adverse supply shock raises the SRAS intercept by 12, while a separate demand expansion shifts AD to \(P=172-2Y\). Find the combined-shock equilibrium and explain why output alone does not reveal the supply shock’s effect. (Answer: The combined equilibrium is \(Y=30,\ P=112\); output returns to its initial value because the demand expansion offsets the supply shock’s output reduction, while the price level is higher.)