Real GDP does not climb in a straight line. It surges, stalls, falls back and picks up again, and it has done this in every economy anyone has ever measured. The pattern has a name, four recognisable stages, and a diagram you will be asked to draw and explain.
📘 What you need to know
The business cycle is the pattern of ups and downs in real GDP over time. This is actual growth.
The long-term trend line shows the economy’s potential growth: what it could produce using its resources fully.
The four stages are boom (peak), slowdown (downturn), recession (trough) and recovery.
A recession is normally defined as two consecutive quarters of negative real GDP growth.
Actual output above the trend is a positive (inflationary) output gap. Below it is a negative (deflationary or recessionary) output gap.
Governments try to smooth the cycle: cooling the economy in a boom and supporting it in a recession.
Output gaps are hard to measure, because nobody knows exactly where potential output sits.
Two different kinds of growth
Keep these two apart in your head, because the whole diagram depends on the difference.
Actual growth is what the economy really produced. It is the wobbly line, and it moves with demand.
Potential growth is what the economy is capable of producing with all its factors of production employed. It is the straight trend line, and it moves only when the quantity or quality of resources changes.
The trend line rises steadily because the economy’s capacity grows year after year. The wobble around it is demand rising and falling faster than capacity does.
The four stages
Stage
What real GDP is doing
What it feels like
Boom (peak)
Growing fast, above trend
Low unemployment, high confidence, rising inflation, firms straining to keep up
Confidence returns, vacancies reappear, spare capacity starts filling up
Watch the wording carefully. A slowdown is not a recession. In a slowdown the economy is still growing, just less quickly. A recession means output is actually falling. Students throw the word recession at any bad news and lose the mark.
Booms and recessions side by side
In a recession
In a boom
Two or more quarters of negative real GDP growth
High and rising rates of real GDP growth
Unemployment rising, vacancies scarce
Unemployment falling, vacancies plentiful
A growing negative output gap and spare capacity
Spare capacity used up, often a positive output gap
Low confidence, so households save and firms postpone investment
High confidence, so firms borrow and take bigger risks
Inflation usually low, sometimes negative
Inflation rising, usually demand-pull
Government spending up, tax revenue down, so the budget deficit grows
Tax revenue up, benefit spending down, so the budget improves
The evaluation point examiners like: the cycle is a model, and models generalise. Not every firm suffers in a recession. Discount supermarkets, repair services and budget brands often do well as households trade down. And the components of AD do not move together: consumption usually recovers well before firms are confident enough to invest again.
Output gaps
An output gap is the distance between actual output and potential output. It is the vertical distance between the two lines on the diagram.
A negative output gap means the economy is producing less than it could. Factories sit idle, workers who want jobs cannot find them, and that lost output can never be recovered. It is gone.
A positive output gap means output has been pushed beyond the sustainable level, usually through overtime and pushing equipment hard. It cannot last, and it puts strong upward pressure on prices.
The measurement problem is real and worth a mark. Actual GDP can be measured, roughly. Potential GDP cannot be observed at all, because it is a judgement about what the economy could be doing. So economists watch the symptoms instead: quickly rising prices suggest a positive gap opening, while rising unemployment and slowing growth suggest a negative one.
Smoothing the cycle
Governments and central banks generally want a gentler cycle, because both extremes are costly. Deep recessions destroy jobs and skills; wild booms build up inflation and bad debt that make the next recession worse.
In a boom: raise interest rates, raise taxes, cut spending growth. Take some heat out of demand.
In a recession: cut interest rates, cut taxes, raise government spending. Support demand until confidence returns.
Some of this happens on its own. In a recession, tax revenue falls and benefit payments rise without any new policy being announced, which cushions incomes automatically. These are the automatic stabilisers, and they are why budget deficits widen in downturns even when a government does nothing.
WORKED EXAMPLE
Real GDP growth in an economy is: Q1 +0.6%, Q2 +0.1%, Q3 −0.3%, Q4 −0.5%. Identify the stage of the cycle in each quarter and state whether the economy entered a recession. [4]
Q1: growth positive and healthyThe economy is expanding, likely near the peak of the cycle.Q2: growth positive but much smallerstill growing, so this is a slowdown, not a recessionQ3 and Q4: growth negative in bothtwo consecutive quarters of negative growthThe economy entered recession in Q4Q3 alone is not enough. The definition needs two in a row, so the recession is only confirmed once Q4 data arrives.
WORKED EXAMPLE
Actual real GDP is $860 billion and potential real GDP is estimated at $900 billion. Calculate the output gap as a percentage of potential output and explain what it means. [3]
Step 1: find the gap860 − 900 = −40Step 2: express it as a percentage of potential(−40 ÷ 900) × 100 = −4.44%A negative output gap of about 4.4%Step 3: explain itThe economy is producing roughly 4.4% less than it could. There is spare capacity, unemployment is likely above its normal level, and inflationary pressure is weak.
💡 Exam tip
Label both lines on the diagram. Actual growth and trend growth, with time on the horizontal axis and real GDP on the vertical.
Mark the stage the question asks about rather than annotating the whole cycle. It is faster and clearer.
Use the two-quarter definition whenever the word recession appears. It is a definition mark waiting to be picked up.
Link stages to other indicators. Unemployment, inflation, confidence and the government budget all move with the cycle, and mentioning them shows range.
Say that potential output cannot be observed. It is the strongest evaluation point available on output gaps.
Do not confuse this diagram with an AD/AS diagram. This one has time on the horizontal axis, not real GDP.
⚠️ Common mix-up
Calling a slowdown a recession. Slower growth is still growth.
Drawing the trend line as flat. It slopes upwards, because capacity grows over time.
Thinking a negative output gap means negative growth. An economy can grow while still sitting below its potential.
Assuming every recession looks the same. Some are shallow and short, some restructure an economy permanently.
Saying a boom is simply good. Booms bring inflation, unsustainable borrowing and a harder landing afterwards.
Treating the cycle as a fixed timetable. There is no rule about how long each stage lasts.
Up next: Does GDP Measure Well-being?, where you take everything you have just learned to measure and start asking what it fails to capture.
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