IB Economics Aggregate Supply Explained

Why Did Everything Get More Expensive? Aggregate Supply & Supply Shocks

Target Question:
"What is aggregate supply in economics and what is the difference between SRAS and LRAS?"

Let's go back to 23 March 2021. The Ever Given - a container ship the length of four football pitches - has run aground and wedged itself diagonally across the Suez Canal, blocking one of the world's most critical trade routes. For six days, billions of dollars of cargo are stuck. Fuel. Electronics. Clothes. Food. Spare parts. All waiting. Economists around the world watch nervously, because they know something the general public doesn't quite grasp yet: when the supply side of the global economy seizes up - even briefly - the consequences spread everywhere. Prices rise. Output falls.

The Ever Given story is a perfect introduction for aggregate supply. It's not about whether people wanted to buy things. Demand was fine - consumers were eager to spend with pandemic savings. The problem was on the supply side: the economy's ability to actually produce and deliver goods had come to a halt. And when that happens, inflation and falling output take over the economy at the same time.

Understanding aggregate supply - why it slopes the way it does, what shifts it, and why economists have been arguing about its long-run shape since at least the 1930s - is one of the most important things you can do as an IB Economics student.

What Is Aggregate Supply?

IB Economics Key Definition - Aggregate Supply (AS): Aggregate supply represents the intended output of goods and services that businesses are prepared and able to provide at various average price levels. It serves as an indicator of the economy's potential output - or total supply - at any given time.

Notice the phrase "intended output" - just as aggregate demand is about planned spending, aggregate supply is about what firms are willing and able to produce. The AS curve shows us this relationship across different price levels.

Short-Run Aggregate Supply (SRAS) - What Firms Can Do Right Now

The short-run aggregate supply (SRAS) curve shows the total anticipated national output from domestic firms across various average price levels in the short run.

In the short run, two things are assumed to be fixed: wage rates and technology. This matters enormously for understanding the shape of the curve.

Why Does the SRAS Curve Slope Upward?
Higher average price levels motivate firms to raise their output. When prices rise, producing more becomes more profitable - so firms increase supply. In the short run, capital is fixed (you can't build a new factory overnight), but firms can adjust variable factors - particularly labour. They can ask workers to do overtime, for instance. This makes the SRAS relatively price elastic in the short run - firms can respond to price changes by adjusting output reasonably quickly.

One more important feature: the slope of the SRAS curve depends on how much spare capacity the economy has. If there are lots of unemployed workers and idle factories, the SRAS curve is relatively flat - firms can ramp up output easily without driving up costs. As the economy approaches full capacity, the curve steepens. There's a direct connection here to the Keynesian model we'll come to later.

What Shifts the SRAS Curve?

A change in any of the main determinants of short-run aggregate supply will shift the SRAS curve - either left (less supply at every price level) or right (more supply at every price level). There are two main categories of determinants: costs of factors of production and indirect taxes.

For costs of production, the IB Economics specifications give you a handy acronym to remember all the key variables:

Labour costs - wages and salaries paid to workers

Interest rates - the cost of borrowing money to finance production

Transportation costs - the cost of moving raw materials and finished goods

Raw material costs - the price of inputs used in the production process

Exchange rate - for firms engaged in international trade, affecting the cost of imported inputs

If any of these costs rise, the SRAS curve shifts to the left - firms can produce less at every price level because their costs have gone up. If costs fall, the SRAS shifts to the right - firms become more willing and able to supply at every price level.

IB Economics Real-life example the 2021–23 Supply Shock: After the pandemic, virtually every letter of the LITRE acronym fired simultaneously - and they all pushed SRAS left. Labour costs (L) surged as workers demanded higher wages and quit in thousands (the "Great Resignation"). Interest rates (I) eventually spiked as central banks fought inflation, raising borrowing costs for businesses. Transportation costs (T) went haywire - shipping container prices rose more than 700% at peak as ports backed up globally. Raw material costs (R) - from timber to semiconductors to cooking oil - all rocketed. And exchange rate (E) volatility added further pressure on import-dependent firms. The result? The SRAS curve shifted sharply left across most major economies: prices rose and output struggled. This is what economists call a negative supply shock - and it was the defining macroeconomic event of 2021–23.

IB Economics Real-life example UK Labour Costs, April 2025: In April 2025, the UK government's increase in employer National Insurance contributions came into force. For many firms, this was a direct and immediate rise in labour costs (L) - shifting SRAS to the left. Businesses across retail, hospitality and care sectors reported cutting staff, reducing hours, and raising prices to absorb the extra costs. A typical leftward SRAS shift playing out in real time: higher costs → less output supplied at every price level → higher average prices.

Indirect Taxes - The Government's Role in Shifting SRAS

Governments impose indirect taxes on producers - environmental taxes (like carbon levies), sales taxes, and excise duties. These add directly to firms' costs of production. Even if firms pass some of the tax on to consumers through higher prices, their profitability is squeezed. The result is a leftward shift of the SRAS curve. When indirect taxes are cut, the SRAS shifts right - costs fall and firms can supply more.

Think of a tax on plastic packaging. Manufacturers suddenly face higher production costs. They either absorb it (reducing profits and potentially output) or pass it on as higher prices. Either way, the economy's aggregate supply is negatively affected, ceteris paribus.

Reading SRAS Shifts on a Diagram

The AD-AS framework:

• Adverse SRAS shift (left): Higher costs of production → SRAS shifts left (SRAS₁ to SRAS₂) → average price level rises (PL₁ to PL₂) → real output contracts (Y₁ to Y₂). This is stagflation - the nightmare combination of rising prices and falling output.

• Favourable SRAS shift (right): Lower costs → SRAS shifts right (SRAS₁ to SRAS₃) → average price level falls (PL₁ to PL₃) → real output expands (Y₁ to Y₃). The good outcome — growth without inflation.

IB Economics Exam Tip - Stagflation: A leftward SRAS shift is the only scenario in the AD-AS model that produces both higher inflation and lower output simultaneously. This is called stagflation. It's why supply-side shocks are so feared by policymakers - conventional monetary policy (raising interest rates) can tackle inflation, but when doing so it worsens the output problem. There's no clean solution. The 1970s oil shocks caused exactly this, as did the 2021–23 post-pandemic period.

What Does Aggregate Supply Look Like in the Long Run?

While economists broadly agree on the shape of the SRAS curve, they have been arguing for nearly a century about what aggregate supply looks like in the long run. This is an important part of the syllabus as the answer determines what governments should do during a recession, how much they should spend, and whether they should intervene at all.

The two main positions are the Monetarist/New Classical view and the Keynesian view.

IB Economics Terminology Note: When referring to the Keynesian long-run AS curve, do NOT call it "LRAS." Call it the Keynesian aggregate supply curve or simply Keynesian AS. The LRAS label belongs exclusively to the Monetarist/New Classical model. Mixing them up will cost you marks.

The Monetarist / New Classical View - "The Economy Sorts Itself Out"

Monetarists (and new classical economists) believe that in the long run, aggregate supply is completely independent of the average price level. In other words, the LRAS curve is perfectly price inelastic - it stands perfectly vertical at the full employment level of output, denoted YF.

Why? Because in the long run, wages and other costs adjust fully. Workers negotiate higher wages when prices rise; input costs recalibrate. Once all these adjustments have happened, the economy returns to producing at its maximum sustainable capacity - YF - regardless of what the price level is doing. The economy, left to its own devices, self-corrects.

The policy implication: If the government tries to boost AD beyond YF - say, through heavy government spending - it won't produce more output in the long run. It will simply push up the average price level. More spending → more inflation. Not more growth. This is the Monetarist critique of Keynesian stimulus: you can't permanently increase output by printing money or running deficits.

The Keynesian View - "The Economy Might Get Stuck"

Keynesians take a fundamentally different view. They believe the AS curve has three distinct sections, reflecting the reality that economies have varying degrees of spare capacity at different points. Crucially, they argue that the economy does not necessarily return to full employment on its own - even in the long run.

Section 1 - Horizontal (Perfectly Price Elastic): When there is massive spare capacity - lots of unemployed workers, idle factories, unused resources - the AS curve is flat. Any increase in AD simply raises real output without affecting the price level at all. You can stimulate the economy for free, in inflation terms.

Section 2 - Upward Sloping: As the economy grows and resources become scarcer, the curve slopes upward. There's pressure on resources; prices start to rise alongside output. Growth happens, but it comes with some inflation.

Section 3 - Vertical (Perfectly Price Inelastic): Once the economy reaches full employment (YF), there is no spare capacity left. Any further increase in AD is purely inflationary - output cannot rise further, so all the extra spending just pushes prices up.

The key Keynesian insight: Economies can get stuck in Section 1 - trapped in recession with high unemployment - not because wages haven't adjusted, but because there isn't enough demand to pull resources into use. In this case, government intervention is not just useful; it's essential.

IB Economics Real-life example Japan's Lost Decades (The Keynesian theory in Action): Japan offers the most powerful real-world example of Keynesian AS in action. From 1995 to 2007, Japan was trapped in prolonged stagnation - nominal GDP actually fell by over 18% during this period. Despite extremely low interest rates and repeated government stimulus packages, the economy remained stuck below its potential output. Consumer and business confidence were so low that even ultra-cheap borrowing couldn't revive private spending. The economy was lodged firmly in Section 1 of the Keynesian AS curve, with enormous spare capacity and deflation - the mirror image of the inflation problem most economies worry about. It's a perfect reminder that economies don't automatically self-correct, as monetarists suggest. Sometimes they just sit there without change.

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Inflationary and Deflationary Gaps - What Happens When the Economy Is Off-Balance?

Whether you're working with the Monetarist LRAS model or the Keynesian AS, you'll need to understand two key concepts: the inflationary gap and the deflationary gap. These describe what happens when an economy's actual output diverges from its potential.

The Inflationary Gap (Positive Output Gap)

IB Economics Key Definition - Inflationary Gap: An inflationary gap (also called a positive output gap) exists when actual real GDP exceeds the full employment level of output: YE > YF. The economy is producing beyond its sustainable capacity. Despite full employment being maintained, the excess demand drives up the average price level - generating demand-pull inflation.

IB Economics Real-life example: the UK post-lockdown boom in mid-2021. Pent-up demand exploded as restrictions lifted. People were spending savings they'd accumulated during lockdown. The economy briefly ran hot - demand was outstripping supply. The result? Rising prices. A positive output gap in action.

The Deflationary Gap (Recessionary Gap / Negative Output Gap)

IB Economics Key Definition - Deflationary Gap: A deflationary gap (also called a recessionary gap or negative output gap) exists when the equilibrium real national output (YE) falls below the full employment level of output (YF): YE < YF. Actual growth is below potential growth. The economy has spare capacity - unemployed workers, idle factories, underutilised resources.

Deflationary gaps are caused by insufficient aggregate demand - specifically, a combination of falling consumer spending, investment expenditure, and/or net export earnings. When AD collapses, the economy slumps below its potential. This is exactly what happened in 2008-09 and again in 2020.

The monetarist and Keynesian schools differ sharply on what to do next. Monetarists argue: be patient, cut wages, reduce costs, and the SRAS will shift right to restore equilibrium without government help. Keynesians argue: don't wait - government spending is needed to fill the demand gap, because private sector recovery may take years, causing enormous unnecessary suffering in the meantime.

Shifting the Long-Run AS - What Makes Economies Grow Over Time?

Whether you're using the Monetarist LRAS model (the vertical line shifts right) or the Keynesian AS model (the whole curve shifts right), economists agree on one thing: aggregate supply does increase over the long run. The question is: what causes it?

There are four main categories of long-run AS shifters:

1. Quantity and/or quality of factors of production More or better-quality land, labour, and capital expands the economy's productive capacity → AS shifts right.

IB Economics Real-life example: Germany's large-scale immigration policy in the 2010s significantly expanded its labour supply, increasing productive potential. Investment in worker training (upskilling) improves labour quality and shifts LRAS right.

2. Improvements in technology Technological advances raise the productivity of all factors → more output from the same inputs → LRAS shifts right.

IB Economics Real-life example: The AI revolution is the defining technological story of the mid-2020s. Goldman Sachs projects AI could boost US productivity by 1.5% annually over the next decade. AI adoption is growing rapidly - the share of firms using AI tools reportedly rose from 20% in 2017 to close to 78% by 2025. If these productivity gains materialise at scale, they would represent one of the most significant rightward LRAS shifts in generations. However, economists remain cautious - Goldman Sachs estimated AI's actual GDP boost in 2025 was close to zero so far. The productivity payoff of major technologies (electricity, the internet) typically takes 20–30 years to fully appear in the data.

3. Increases in efficiency When resources are deployed more effectively - less waste, better processes, improved logistics - the economy produces more for the same cost → LRAS shifts right.

IB Economics Real-life example: Supply chain efficiency gains following COVID-19 (rerouting, automation, diversification) gradually restored productive capacity in many economies. Firms that adopted lean manufacturing or digital supply chain management found they could produce more with the same inputs.

4. Changes in institutions Better legal systems, financial markets, education, and healthcare all increase productive potential → LRAS shifts right.

IB Economics Real-life example: Rwanda's dramatic improvements in governance, infrastructure, and education since the 1990s have shifted its productive capacity significantly rightward. Conversely, erosion of institutional quality (corruption, weak rule of law) shifts LRAS left - as seen in Venezuela over the past two decades.

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IB Economics Real-life example: AI and the LRAS Debate: On one hand, AI investment has been enormous: global AI-related capital expenditure has risen from around $60 billion in Q4 2023 to $255 billion by Q2 2025. On the other hand, most macro-studies of productivity growth find limited evidence of significant AI-driven gains so far. The San Francisco Federal Reserve noted that even firms that say AI is useful see little evidence of transformative gains yet. Some researchers compare AI to electricity: the productivity payoff of electrification didn't materialise until about 30 years after the invention, once businesses had fully reorganised around the new technology. The honest answer is: we don't know yet. But if the optimistic projections are right, the rightward LRAS shift from AI could be the most significant in decades.

IB Economics Summary:

The Monetarist vs Keynesian debate over aggregate supply is a current disagreement that shapes real policy decisions right now. When a government decides whether to run a budget deficit during a recession, it's implicitly making an assumption about where the economy sits on the AS curve and whether it will self-correct. When a central bank raises interest rates to fight inflation, it's acting on a model of how supply and demand interact in the short run versus the long run.

The 2021–23 global inflation surge was an obvious test case. Monetarists pointed to excessive money supply growth as the culprit - too much AD chasing too little AS. Keynesians emphasised the supply-side nature of the shock - the pandemic, the Ever Given, the Ukraine war, the semiconductor shortage - arguing that demand-side policies (like rate hikes) were the wrong tool. In the end, both camps had a point. The truth was a lot more complex than either model alone could describe. Which is, of course, exactly what your IB Economics examiner wants you to write to obtain full marks.

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Frequently Asked Questions

What is aggregate supply in economics?

Aggregate supply (AS) is the intended total output of goods and services that businesses in an economy are willing and able to produce at various average price levels. It serves as an indicator of the economy's productive capacity. In the short run, the SRAS curve slopes upward - higher prices incentivise more output. In the long run, economists disagree: Monetarists see a vertical LRAS at full employment (YF), while Keynesians see a three-section curve reflecting varying degrees of spare capacity.

What is the LITRE acronym in IB Economics?

LITRE is an IB Economics acronym for the key determinants of the SRAS curve through costs of factors of production: Labour costs, Interest rates, Transportation costs, Raw material costs, and Exchange rates. If any of these costs rise, the SRAS curve shifts to the left (less supply at every price level, higher inflation, lower output). If costs fall, the SRAS shifts right (more supply, lower prices, higher output).

What is the difference between the Monetarist LRAS and the Keynesian AS curve?

The Monetarist (New Classical) LRAS is a vertical line at the full employment level of output (YF), indicating that aggregate supply is perfectly price inelastic in the long run and independent of the price level. The economy self-corrects to YF regardless of AD. The Keynesian AS curve has three sections: a horizontal section (plenty of spare capacity - AD boosts output without inflation), an upward-sloping section (growing pressure on resources), and a vertical section at YF (full capacity - any extra AD is purely inflationary). Critically, Keynesians argue the economy does not automatically reach YF - it can get stuck in recession without government intervention.

What is an inflationary gap and a deflationary gap?

An inflationary gap (positive output gap) occurs when actual real GDP exceeds the full employment level (YE > YF) - the economy is overheating, creating demand-pull inflation. A deflationary gap (recessionary gap or negative output gap) occurs when actual real GDP falls below the full employment level (YE < YF) - the economy has spare capacity and unemployment, caused by insufficient aggregate demand. Monetarists argue market forces will restore equilibrium; Keynesians argue government intervention is needed.

What shifts the Long-Run Aggregate Supply (LRAS) curve?

The LRAS curve (or Keynesian AS in the long term) shifts right - increasing potential output - due to four main factors: (1) increases in the quantity and/or quality of factors of production (more labour, capital, or land); (2) improvements in technology, which raise productivity; (3) increases in efficiency, ensuring resources are used more effectively; and (4) improvements in institutions, such as better education, healthcare, legal systems, and financial markets. These factors expand the economy's productive capacity over the long run.

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