Output Gaps, Potential Output & Shocks

The speed an economy can sustain

The previous guide gave you the AD-AS diagram and that vertical line standing at potential. Now we put that line under the microscope, because almost every macroeconomic argument you will ever hear turns on it. [[potential-output|Potential output]] is the level of real GDP an economy can produce when it uses its labour, capital, and technology at their normal, sustainable rate — not the absolute maximum, but the comfortable cruising speed. It is the same idea as the vertical [[long-run-aggregate-supply|long-run aggregate supply]] line: what the economy can make once wages and prices have had time to adjust, independent of the price level.

The word "sustainable" is doing a lot of work, so take it seriously. An economy can sprint past potential for a while — factories run extra shifts, almost everyone who wants a job has one, machines never cool down. But that pace is borrowed, not owned: it overworks people and equipment and, as you saw in the inflation rung, it tends to bid up wages and prices. Potential output is the line beyond which going faster buys you inflation rather than lasting extra production. Think of it like a runner's marathon pace versus a sprint — the sprint is real, but try to hold it for a whole race and something breaks.

The output gap: running hot or running cold

The [[output-gap|output gap]] is simply the distance between where the economy actually is and that potential line, written as a percentage of potential. You met it briefly in the AD-AS guide; here is the arithmetic. If actual GDP is 1,020 and potential is estimated at 1,000, the gap is (1,020 − 1,000) ÷ 1,000 = +2%: a positive gap, the economy running hot. Flip it — actual 960 against potential 1,000 — and the gap is −4%: a negative gap, the economy running cold. The sign tells you the whole story, and each sign comes with its own unmistakable symptoms.

Output gap = (Actual GDP - Potential GDP) / Potential GDP

  Actual 1020, Potential 1000  ->  +2%   POSITIVE gap (hot)
      symptoms: low unemployment, overtime, rising inflation

  Actual  960, Potential 1000  ->  -4%   NEGATIVE gap (cold)
      symptoms: cyclical unemployment, idle capacity, weak prices

A positive gap is a boom that has overshot: unemployment dips below its natural rate, firms scramble for scarce workers and materials, and that competition pushes wages and prices up — the economy overheating. A negative gap is the painful one: output sits below what the country could make, factories stand idle, and the slack shows up as [[recession|recession]] and cyclical unemployment, real people out of work not because their skills vanished but because demand fell away. Mapping it back to the diagram: a positive gap is actual output to the right of the LRAS line, a negative gap to the left. The business cycle from the first guide is, in this language, the gap swinging between plus and minus.

Two kinds of shock, two different stories

What knocks the economy off its potential line in the first place? A shock — an unexpected jolt that shifts one of the AD-AS curves — and it matters enormously which curve gets hit. A [[demand-shock|demand shock]] moves aggregate demand: a wave of pessimism, a credit crunch, a collapse in investment all drag AD leftward, while a confidence boom or a spending surge shoves it right. As you saw last time, a demand shock pushes output and the price level the same direction — a slump arrives hand in hand with falling or slowing inflation, a boom with rising inflation.

A [[supply-shock|supply shock]] is different in kind. It moves the [[short-run-aggregate-supply|short-run aggregate supply]] curve by changing the cost or availability of producing things: an oil-price spike, a crop failure, a war that severs a supply chain, a pandemic that empties factories. An adverse supply shock — costs jumping — shifts SRAS leftward, and the result is the ugly mirror image of a demand shock: output and the price level move in opposite directions. Production falls while prices rise. That combination of stagnation plus inflation has its own dreaded name, [[stagflation|stagflation]], and it is exactly what the oil crises of the 1970s delivered.

Why a supply shock forces a painful choice

Here is the deepest payoff of telling the two shocks apart. After a demand shock, the economy has only one problem at a time. A negative demand shock leaves output too low and inflation too low — both pulling the same way. So in principle a single lever helps both: push aggregate demand back rightward and you raise output and prices together, easing both troubles at once. The pain of a demand-driven slump is real, but the prescription is at least pointing in one consistent direction.

An adverse supply shock springs a cruel trap instead, because it inflicts two problems that pull opposite ways: output is too low and inflation is too high at the same time. Now reach for the same demand lever and watch it betray you. Boost demand to rescue jobs and output, and you pour fuel on already-rising prices — inflation gets worse. Tighten demand to tame inflation, and you deepen the slump — output and jobs get worse. There is no single move that fixes both with demand-side policy; every choice trades one pain for another.

That dilemma is precisely why the 1970s broke the confident postwar consensus and why central bankers still lose sleep over supply shocks. There is no painless exit: a policymaker must decide which evil to fight first, knowing the other will get worse. The honest deeper answer is that a true supply shock is not really a demand problem at all — it has lowered what the economy can actually produce — so the lasting cure lies on the supply side (new energy sources, repaired supply chains, productivity), which is slow. Demand-side policy can only choose how to share out the short-run pain, not erase it. Hold on to that frustration; it is the reason the policy rungs ahead are full of hard trade-offs rather than magic buttons.

Setting up the policy rungs

Step back and you can see the whole next stretch of the ladder laid out. Diagnose the gap and the shock, then the question becomes what, if anything, to do. One school says a negative gap can linger for years if wages are sticky downward, so the government or central bank should step in and shove AD back toward potential — the case for active demand management that the fiscal-policy and monetary-policy rungs build out in detail. Another school warns that policymakers are slow and fallible, that they may simply mistime the push and overshoot, and that the economy often self-corrects faster than feared. You do not have to settle that fight today; you just need to see that it is a real fight, fought over the very gap-and-shock framework you now hold.

1. Estimate the output gap: compare actual GDP with potential — positive (hot) or negative (cold)?
2. Diagnose the shock: did output and prices move the same way (demand) or opposite ways (supply)?
3. Read the trade-off: a demand shock points one way; a supply shock forces a choice between fighting the slump and fighting inflation.
4. Only then weigh policy — fiscal and monetary tools, and whether to use them at all — which the next rungs unpack.