Aggregate Supply

Aggregate supply (AS) is the total quantity of goods and services all producers in an economy are willing and able to supply at a given price level, over a given time period — the mirror image of aggregate demand, and together the two form the whole macroeconomic equilibrium diagram. Like every supply-and-demand model on this course, a change in the economy's own price level causes a movement along a fixed AS curve; a change in anything else — costs, capacity, technology — causes a shift of the whole curve to a new position. The spec splits AS into two genuinely different curves, not two versions of the same one: short-run aggregate supply (SRAS), which responds to costs the economy is currently stuck with, and long-run aggregate supply (LRAS), which responds only to the economy's underlying productive capacity. Confirmed directly in examiner reports checked for this course: confusing which curve a question is asking about — or drawing the right curve and simply labelling it wrong — is one of the most consistently lost sets of marks on this entire diagram.

SRAS is upward-sloping: at a higher price level, firms supply more output, because in the short run at least one major cost — usually wages, fixed by an existing contract — hasn't caught up yet (the mechanism block below derives exactly why). The spec names three influences that shift SRAS, and all three work the same way: they change the cost of producing a given unit of output, at every price level, without changing the economy's underlying capacity to produce at all. A rise in raw material or energy costs (an oil-price shock is the standard real-world exam context) raises unit costs directly, shifting SRAS left — and a fall in raw-material or energy costs (2018's oil-price fall, tested directly as a WEC12 Section B diagram question, is the standard example) shifts SRAS right, the exact reverse. A depreciation of the exchange rate raises the domestic-currency cost of any imported input, which shifts SRAS left for an economy that depends on imports — and does the exact opposite on an appreciation. A rise in the tax rate on production (or on the inputs firms buy) raises unit cost the same way a raw-material price rise does; a tax cut works in reverse — and it's worth flagging early that 'tax' shows up again under LRAS below, through a completely different mechanism (see the trap below).

LRAS asks a different question entirely: not 'what will firms supply at this price level given today's costs', but 'what CAN the economy produce once every cost, including wages, has had time to fully adjust'. Two models give two different answers to what that curve looks like, and the difference is the conceptual centre of this lesson: the classical model draws LRAS as a single vertical line at the economy's potential output, completely unaffected by the price level; the Keynesian model draws it as three joined sections — flat, then upward-sloping, then vertical — because it assumes an economy can genuinely sit below potential output for a sustained period, with real spare capacity that the classical model assumes gets bid away almost immediately. Both shapes are accepted on this paper — which one to draw is a modelling assumption to state explicitly, not a fact to memorise as universally 'correct'.

Because LRAS represents capacity rather than cost, only a change to the economy's actual productive potential shifts it — and the spec names six sources, all genuinely different from SRAS's three: technology (a new production method raises output from the same inputs), productivity (more output per worker or per unit of capital, whatever the underlying cause), education and skills (a more capable workforce produces more from the same headcount), government regulation and tax (deregulation and investment-focused tax changes can raise the economy's actual capacity to produce, not just the incentive to produce more of today's output), demography and net migration (a larger working-age population is a literal increase in the labour input available), and competition policy (breaking up an inefficient monopoly forces it to face rivals again, which can reduce X-inefficiency — the waste that builds up when a firm insulated from competition operates above its true minimum-cost curve instead of being disciplined into it — and raise the economy's output at every price level, not just redistribute who earns it). None of these six changes what firms are willing to supply at today's cost structure — they change what the economy is capable of producing at all, which is exactly why they shift LRAS and not SRAS. Net migration is worth flagging as the one influence on this list whose SIGN genuinely changes which way it shifts the curve, not just how far: net migration itself is immigration minus emigration, so a sustained rise — working-age migrants joining the labour force — is a literal increase in the labour input available and shifts LRAS right, exactly as above, while a sustained fall (net emigration, and especially the loss of already-skilled workers) is a literal fall in that same input and can drag down average productivity too, shifting LRAS left with no change at all to raw-material costs, exchange rates, or tax rates. A real mark scheme rewards precisely this second, decreasing direction: for a country whose net migration swung from positive to sharply negative, analysis credit was available for "reduction in productive capacity/fall in factors of production" and, as a distinct second point, "if skilled labour leaves, productivity may fall" (January 2020 MS, Q7) — and the paired examiner report is explicit that describing migration's effects on the economy in general, without ever stating what happens to the LRAS curve itself, earned no analysis credit at all, however accurate that general discussion was (January 2020 ER, Q7). (This is spec point 2.3.3.3's own named list of LRAS shifters — a different spec point, asking a different question, from 2.3.5's five named causes of potential growth, covered in full in the dedicated Growth Theory and Output Gaps lesson: investment/FDI, innovation, labour force growth/migration, competition, and productivity. The two lists don't map one-to-one — investment doesn't get its own named entry here, and technology, education/skills and regulation/tax don't get their own named entry there — because 'what shifts LRAS' and 'what causes potential growth' are the same underlying economics described from two different spec points, not two competing attempts at one list that happen to disagree.)

Every mechanism this lesson has derived reduces to one question, asked of any policy or event: does it change what today's inputs cost, or does it change what the economy is capable of producing at all? The three SRAS influences (raw-material and energy costs, the exchange rate, the tax rate on production) all answer 'cost' — they raise or lower unit cost at an unchanged capacity, which is why they shift a curve that itself represents what firms currently choose to supply at today's cost structure, not a ceiling. The six LRAS influences answer 'capacity' — none of them touch what a firm's inputs cost today; they change the labour, capital, technology, or competitive discipline available to the whole economy, which is why they shift a curve that represents a ceiling rather than a choice. The tax-rate/tax-policy trap named above is the clearest single test of whether that distinction has actually been understood, precisely because the same word ('tax') sits on both lists for two unrelated reasons — a mark scheme rewards naming which channel is in play, not just naming that a tax changed.

None of this happens in isolation from AD. The equilibrium price level and output an economy actually settles at is wherever AD meets AS — SRAS in the short run, LRAS once wages have fully adjusted — which is exactly why a rise in real GDP can come from two genuinely different sources that look identical on a single year's growth figure: AD rising against an unchanged LRAS (using capacity the economy already had), or LRAS itself shifting outward (growing that capacity). The dedicated Growth Theory and Output Gaps lesson builds directly on every mechanism derived here to formalise that distinction as actual versus potential growth — the same cost-side/capacity-side question this lesson has been asking of SRAS and LRAS individually, now asked of a rise in output itself.