Why Stagflation Is an AD-AS Puzzle
In the short-run aggregate demand and aggregate supply model, a change in demand and a change in supply can move output and the price level in different combinations. That distinction matters when prices rise: stronger aggregate demand can raise the price level while expanding output, whereas an adverse supply shock can raise the price level while contracting output. The second pattern is the central AD-AS explanation of stagflation.
The word combines stagnation and inflation. In this tutorial, the term refers to an outcome with a higher price level and lower real output than the starting equilibrium. It does not, by itself, establish that prices will keep rising over time. Nor does the basic AD-AS diagram calculate an unemployment rate. It identifies the direction of output and price-level changes under stated assumptions.
From an Adverse Supply Shock to Stagflation
An adverse short-run aggregate supply shock raises firms’ costs or otherwise reduces the amount they are willing to produce at a given price level. Examples in a hypothetical economy could include a sudden increase in energy costs or a disruption that makes key inputs harder to obtain. In the AD-AS diagram, short-run aggregate supply (SRAS) shifts left, or equivalently upward. Aggregate demand (AD) is held fixed.
At the original price level, firms now offer less output than before. The new intersection of AD and SRAS occurs at lower output and a higher price level. The higher price level is not caused by the economy moving along a fixed SRAS curve; it reflects a shift of the entire SRAS curve. The direction of the result follows from the slopes of the two curves: AD slopes downward and SRAS slopes upward in the standard short-run model.
Worked Example: An Input-Cost Shock Raises Prices as Output Falls
In a hypothetical economy, let the initial AD curve be \(P=140-2Y\), and let initial SRAS be \(P=20+Y\). Here \(P\) is an index of the price level and \(Y\) is real output measured in consistent index units. An adverse input-cost shock shifts SRAS to \(P=35+Y\), while AD does not change. Calculate both equilibria and classify the result.
Method and assumptions. Treat both equations as linear short-run curves over the relevant range. Equilibrium is the point where the price level on AD equals the price level on SRAS. The demand curve is downward sloping, the supply curve is upward sloping, and only SRAS shifts.
Initial equilibrium. Set the original curve equations equal:
Subtract 20 from both sides and add \(2Y\) to both sides: \(120=3Y\), so \(Y=40\). Substitute into either curve: \(P=140-2(40)=140-80=60\). The initial equilibrium is \((Y,P)=(40,60)\). Checking SRAS gives \(20+40=60\), as required.
Equilibrium after the shock. Use the shifted SRAS curve:
Subtract 35 and add \(2Y\): \(105=3Y\), so \(Y=35\). Then \(P=140-2(35)=140-70=70\). Checking shifted SRAS gives \(35+35=70\).
Output changes by \(35-40=-5\), while the price level changes by \(70-60=10\). Prices rise as output falls, so this is a stagflationary outcome under the stated one-shock assumptions. The common mistake to avoid is to call it an AD shift: the equation for AD stayed fixed; it was the supply intercept that rose.
Why the Direction of the Changes Follows
A simple linear model makes the comparative statics explicit. Suppose AD is \(P=A-bY\) and SRAS is \(P=C+sY\), where \(A\) and \(C\) are intercepts and \(b>0\), \(s>0\) are the absolute slope of AD and the slope of SRAS. These signs encode the standard downward-sloping AD and upward-sloping SRAS curves. Equilibrium requires the two expressions for \(P\) to be equal.
At equilibrium, \(A-bY=C+sY\), so \((b+s)Y=A-C\).
Because \(b+s>0\), equilibrium output is \(Y=(A-C)/(b+s)\).
An adverse supply shift raises \(C\) by \(\Delta C>0\), with \(A\), \(b\), and \(s\) fixed. The change in output is \(\Delta Y=-\Delta C/(b+s)<0\).
Along the unchanged AD curve, \(\Delta P=-b\Delta Y=b\Delta C/(b+s)>0\). Thus output falls and the price level rises.
This derivation shows why the result is not merely a diagram-reading convention. A higher supply intercept subtracts from equilibrium output because the curves must still intersect, and the lower output on the unchanged AD curve corresponds to a higher price level. The conclusion depends on the model’s assumptions, including \(b>0\), \(s>0\), and no simultaneous shift in AD. It need not describe every real-world episode, especially when several forces change at once.
Demand-Driven Price Increases Are Different
A rightward shift of AD raises planned spending at each price level. With SRAS fixed and upward sloping, the new equilibrium has both higher output and a higher price level. This is a demand-driven increase in prices, but it does not produce the defining output decline of the supply-shock stagflation pattern.
Worked Example: Distinguishing a Demand Expansion from Stagflation
Use a hypothetical economy with initial AD \(P=100-Y\) and SRAS \(P=10+Y\). A demand expansion shifts AD to \(P=112-Y\). SRAS is unchanged. Find the initial and new equilibria, then decide whether the outcome is stagflationary.
Method and assumptions. Equate AD and SRAS at each equilibrium. The curves are linear, AD slopes downward, SRAS slopes upward, and the only change is the AD intercept.
Initial equilibrium. Set \(100-Y=10+Y\). Then \(90=2Y\), so \(Y=45\). Substitution gives \(P=100-45=55\); SRAS confirms \(10+45=55\).
New equilibrium. Set the shifted AD equal to unchanged SRAS: \(112-Y=10+Y\). Then \(102=2Y\), so \(Y=51\). Substitution gives \(P=112-51=61\); SRAS confirms \(10+51=61\).
Output rises by \(51-45=6\), and the price level rises by \(61-55=6\). This is not the stagflationary pattern: prices rise, but output rises rather than falls. The common mistake is to label any increase in the price level “stagflation.” The direction of output must also be checked.
The contrast can be summarized by holding one curve fixed at a time. These directional results are for the standard short-run model, not universal claims about every combination of economic changes.
| Isolated change | Price level | Real output | Interpretation |
|---|---|---|---|
| AD shifts right; SRAS fixed | Rises | Rises | Demand-driven expansion and higher prices |
| SRAS shifts left; AD fixed | Rises | Falls | Stagflationary supply-shock pattern |
| SRAS shifts right; AD fixed | Falls | Rises | Favorable supply-shock pattern |
Size of the Effect and a Calculation Trap
The direction of a supply-shock effect follows from the curve slopes, but its size depends on those slopes. In the linear model above, a supply-intercept increase of \(\Delta C\) reduces output by \(\Delta C/(b+s)\) and raises the price level by \(b\Delta C/(b+s)\). A steeper AD curve, represented by a larger \(b\), changes how much of the shock appears as a price-level change relative to the output change. These formulas describe the specified linear model; they should not be applied to curves whose equations or relevant ranges differ.
Worked Example: Quantifying a Supply Shock with Different Slopes
Suppose a hypothetical economy has AD \(P=180-3Y\) and SRAS \(P=20+Y\). A cost shock shifts SRAS to \(P=35+Y\), leaving AD fixed. Find the changes in output and the price level, and verify them using the slope-based result.
Method and assumptions. First solve the two curve intersections directly. Then use the linear comparative-statics formula with \(b=3\), \(s=1\), and \(\Delta C=15\). Output is measured in index units and \(P\) is a price-level index.
Initial equilibrium. Set \(180-3Y=20+Y\). Thus \(160=4Y\), so \(Y=40\). The price level is \(P=180-3(40)=180-120=60\), also \(20+40=60\).
New equilibrium. Set \(180-3Y=35+Y\). Thus \(145=4Y\), so \(Y=36.25\). The price level is \(P=180-3(36.25)=180-108.75=71.25\). Shifted SRAS gives \(35+36.25=71.25\), confirming the intersection.
Output changes by \(36.25-40=-3.75\), and the price level changes by \(71.25-60=11.25\). The formula gives \(\Delta Y=-15/(3+1)=-3.75\) and \(\Delta P=3(15)/(3+1)=11.25\), matching the direct calculation. The mistake to avoid is dividing the shock by \(b\) alone: both slopes enter the equilibrium denominator \(b+s\).
Interpreting the Result Carefully
In an AD-AS analysis, an outcome is best described by stating both movements and the assumed cause: for example, “With AD fixed, an adverse SRAS shift raises the price level and reduces real output.” This is more informative than saying only that “inflation occurred,” because the latter does not indicate whether output expanded or contracted.
The diagram also has limits. A leftward SRAS shift is a model of a supply-side disturbance, not proof that a particular observed episode had only that cause. If AD also shifts, the net changes depend on both shifts. The isolated-shock comparison taught here does not determine the result of simultaneous shifts. Similarly, a short-run price-level change is not enough to establish a continuing rate of inflation. Keep the model’s time horizon and assumptions attached to the conclusion.
- Check the curve that shifts. A supply shock moves SRAS; a demand shock moves AD.
- Track both equilibrium variables. A higher price level alone does not establish stagflation.
- Use the fixed curve for the final comparison. With AD fixed, lower equilibrium output lies at a higher price level on AD.
- State the assumptions. The clean directional result assumes the other curve does not shift.
Check Your Understanding
For each item, use the stated linear short-run curves and report the equilibrium directions or values requested.
- AD is \(P=130-2Y\) and initial SRAS is \(P=10+Y\). SRAS shifts to \(P=25+Y\), with AD fixed. Find both equilibria and classify the change. (Answer: Initially \(Y=40, P=50\); afterward \(Y=35, P=60\); output falls and the price level rises.)
- AD shifts from \(P=96-Y\) to \(P=108-Y\), while SRAS remains \(P=16+Y\). Find the initial and new equilibria and state whether the result is stagflationary. (Answer: Initially \(Y=40, P=56\); afterward \(Y=46, P=62\); not stagflationary because output rises.)
- AD is \(P=150-2Y\), and SRAS shifts from \(P=30+Y\) to \(P=39+Y\). Calculate the change in equilibrium output and the price level. (Answer: Output falls by 3 units, from 40 to 37; the price level rises by 6 index points, from 70 to 76.)
- AD is \(P=200-3Y\), and SRAS shifts from \(P=20+Y\) to \(P=36+Y\), with AD unchanged. Calculate both equilibria and verify the directions. (Answer: Initially \(Y=45, P=65\); afterward \(Y=41, P=77\); output falls by 4 and the price level rises by 12.)
- AD is \(P=120-2Y\), and SRAS shifts from \(P=20+2Y\) to \(P=32+2Y\). Find the initial and new equilibria. (Answer: Initially \(Y=25, P=70\); afterward \(Y=22, P=76\); output falls by 3 and the price level rises by 6.)