Does Government Spending Really Revive the Economy? The IS-LM Model, Crowding Out, and Lessons from the U.S. and U.K.

Expansionary fiscal policy—raising government spending or cutting taxes to pull an economy out of a slump—lifts national income (Y), but in the IS-LM model it also nudges the interest rate (r) higher, which pushes some private investment out of the picture. Economists call that squeeze crowding out. How big it gets depends on the slope of the LM curve, on whether the central bank leans with or against the stimulus, and, once trade and capital flows enter, on the exchange-rate regime. This guide builds the model step by step the way an intermediate macroeconomics course does, then stress-tests it against what actually happened in the United States and the United Kingdom between 2008 and 2023.

What do the IS and LM curves actually represent?

John Hicks condensed Keynes's General Theory into a single diagram in 1937, and it has anchored undergraduate macro ever since. Put real output (Y) on the horizontal axis and the real interest rate (r) on the vertical axis. Every point where the goods market clears sits on the IS curve; every point where the money market clears sits on the LM curve. The two markets are not independent: income changes the demand for money, and the interest rate changes spending, so the economy settles only where both conditions hold at once.

Why does the IS curve slope downward?

IS stands for investment equals saving. When borrowing costs fall, more factory upgrades, software projects, and housing starts clear the hurdle rate, so investment rises. That extra spending ripples through the economy via the Keynesian multiplier and raises equilibrium income. Higher rates do the reverse, so the curve slopes down. A change in government purchases (G) or taxes (T) does not move you along the curve—it shifts the whole curve, because it changes demand at every interest rate.

Why does the LM curve slope upward?

LM stands for liquidity preference equals money supply. Households and firms hold money to pay for transactions and, at the margin, as an alternative to bonds. When income rises, transactions demand rises too. If the real money supply (M/P) is fixed, people try to sell bonds to get cash, bond prices fall, and yields rise until money demand is choked back to the available supply. Higher income therefore pairs with a higher equilibrium rate. For the full algebra, see the IS-LM model entry in the CampusDown Wiki, which derives both curves and their slopes.

Put the two together and the short-run equilibrium is the single intersection where planned spending equals production and money demand equals money supply. Every policy question in the rest of this guide is really a question about which curve moves, how far, and how steep the other curve is.

Why does fiscal stimulus crowd out private investment?

Suppose Congress or Parliament funds a large highway and broadband program. New public demand shifts the IS curve to the right. If interest rates stayed put, output would rise by the full multiplier. But as income climbs, firms and households need more money for transactions, and with an unchanged money supply the market for loanable funds tightens. The Treasury is also issuing more bonds to pay for the program, adding supply to the bond market. Yields rise.

Higher borrowing costs make some private projects unprofitable. Here is a round-number illustration: the government adds $100 billion in spending, the resulting rise in rates cancels $30 billion of business investment, and output ends up roughly $70 billion higher instead of the full amount. Part of the public stimulus has simply displaced private spending. The comparison below shows why fiscal and monetary expansions both raise output yet move interest rates and investment in opposite directions.

Both tools push output up, yet one raises borrowing costs and the other lowers them, so the right choice depends on what is causing the slump and on where interest rates already are when the shock arrives.

Skimming the section headings first makes a long guide much easier to map out. Try writing down one question of your own as you read.

When does crowding out disappear, and when is it total?

The size of crowding out is a question about slopes. At one extreme sits the liquidity trap: when short-term rates are already near zero, people are happy to hold extra money instead of bonds, the LM curve is nearly flat, and a fiscal expansion raises income without lifting rates. Nothing gets crowded out. That is why Keynes argued for public works during the Great Depression; the reasoning is laid out in the Keynesian economics wiki entry.

At the other extreme is the classical case. If money demand does not respond to interest rates at all, the LM curve is vertical. Fiscal stimulus then raises only the interest rate, and every dollar of public spending displaces a dollar of private spending—complete crowding out. Real economies live between these poles, which is why governments and central banks often coordinate. If the central bank expands the money supply while the Treasury spends, LM shifts right alongside IS and rates hold steady. That accommodating policy mix delivers the largest output gain, though it can also stoke inflation once the economy reaches capacity.

The practical lesson is that the same deficit can be highly effective in one year and largely self-defeating in another, depending on where the policy rate sits and how the central bank chooses to respond.

What does the U.S. and U.K. policy record tell us?

Textbook diagrams become far more convincing once you map them onto real episodes. The last fifteen years gave American and British students an unusually clean natural experiment: one period where the LM curve was almost flat, one where fiscal and monetary policy pushed in the same direction, and one where they pulled hard against each other.

2008–2009: ARRA at the zero lower bound

By December 2008 the Federal Reserve had cut the Fed funds target range to 0–0.25%, and the Bank of England took Bank Rate to 0.5% in March 2009 while launching quantitative easing. In February 2009 the American Recovery and Reinvestment Act committed roughly $787 billion (later re-estimated by the Congressional Budget Office at about $830 billion) in spending and tax relief. Deficit hawks warned that Treasury borrowing would send yields soaring. It did not: the 10-year Treasury yield stayed below 4% through the recovery. In IS-LM terms the economy was sitting on the flat segment of LM, private demand for credit had collapsed, and the Fed was absorbing bonds. Interest-rate crowding out was close to zero, which is why many economists argue the real mistake was making the stimulus too small and withdrawing it too early.

2020–2021: a fiscal surge alongside QE

The pandemic response was a textbook accommodating policy mix. Congress passed the CARES Act (about $2.2 trillion) in March 2020 and the American Rescue Plan ($1.9 trillion) in March 2021, while the U.K. Treasury ran the furlough scheme. At the same time the Fed returned to zero rates and open-ended asset purchases, roughly doubling its balance sheet to near $9 trillion, and the Bank of England cut to 0.1% and expanded QE. Both IS and LM shifted right together. The 10-year Treasury yield fell to around 0.5% in mid-2020 despite record deficits, so crowding out never materialized; the cost showed up later as demand outran constrained supply.

2022–2023: tightening and the return of crowding-out worries

With U.S. CPI inflation peaking at 9.1% in June 2022, the Fed raised rates from near zero to 5.25–5.50% by July 2023, and the Bank of England lifted Bank Rate to 5.25%. Now the central bank was shifting LM left while deficits stayed large, the opposite of accommodation. The 10-year Treasury yield briefly topped 5% in October 2023, mortgage rates hit their highest levels in two decades, and rate-sensitive business investment and housing slowed—classic crowding out. Britain added a vivid footnote: the September 2022 mini-budget announced unfunded tax cuts just as the Bank was tightening. Gilt yields spiked, pension funds using liability-driven investment strategies faced margin calls, the pound fell to a record low near $1.04, and the Bank had to buy long-dated gilts temporarily to restore order.

Read side by side, these three episodes show that the response of interest rates to a deficit is not fixed. It depends on slack in the economy, on the central bank's reaction function, and on whether markets trust the fiscal plan behind the borrowing.

How does the Mundell-Fleming model change the answer in an open economy?

The closed-economy story ignores the fact that capital flows across borders in seconds. The Mundell-Fleming model adds a third curve, the BP curve, which marks combinations of income and interest rates where the balance of payments is in equilibrium. With perfect capital mobility, BP is horizontal at the world interest rate: if domestic yields rise even slightly above global yields, foreign money pours in.

Floating vs. fixed exchange rates

Under a floating exchange rate, which is how the dollar and sterling trade, a fiscal expansion raises domestic rates, draws in foreign capital, and appreciates the currency. A stronger currency makes exports pricier and imports cheaper, net exports fall, and the IS curve drifts back left. Fiscal policy loses much of its punch, while monetary easing gains extra force because a weaker currency boosts exports. The early 1980s are the standard illustration: large Reagan-era deficits combined with Paul Volcker's tight money drove the dollar sharply higher, widened the trade deficit, and ultimately prompted the 1985 Plaza Accord to talk the dollar down.

Under a fixed exchange rate, the logic flips. When fiscal stimulus attracts capital, the central bank must buy foreign currency and issue domestic money to hold the peg, which shifts LM right as well, so fiscal policy becomes very powerful. Monetary expansion, by contrast, triggers capital outflows that the central bank must offset by selling reserves, so it achieves almost nothing.

The U.K.'s 2022 episode is a reminder of the model's limits. Mundell-Fleming assumes credible policy. When investors doubt fiscal sustainability, they demand a higher risk premium, the effective BP curve shifts up, and the currency can weaken even while yields climb. Always ask whether the market is pricing higher returns or higher risk.

Where do students lose marks on IS-LM exam questions?

On intermediate macro midterms and AP or A-level style essays, the errors are rarely about memorizing equations. They come from skipping a link in the causal chain, especially the interest-rate step in a closed economy and the exchange-rate step in an open one.

Three classic mistakes

• Movement along vs. shift of a curve: a change in r moves the economy along IS or LM, while changes in G, T, or M shift the whole curve.
• Mixing up interest-rate directions: both expansions raise Y, but fiscal expansion raises r and monetary expansion lowers it.
• Dropping the currency link: higher domestic rates, then capital inflows, then appreciation, then lower net exports and a leftward IS shift. Skip one step and the floating vs. fixed conclusions come out backward.

Consider a typical exam prompt: "A small open economy with a floating exchange rate and perfect capital mobility passes a $500 billion infrastructure bill. Using the Mundell-Fleming model, explain the effect on output, the interest rate, the exchange rate, and the trade balance." A student who writes only "IS shifts right, r rises, some investment is crowded out, Y increases" will lose most of the credit. The full answer notes that the domestic rate cannot stay above the world rate, capital inflows appreciate the currency, net exports fall, and IS returns toward its original position, leaving output roughly unchanged while the deficit is mirrored by a wider trade deficit.

Before handing in a graph or essay, run through this checklist:

• Did I identify which curve the policy shifts first, and in which direction?
• Did I explain how the change in income alters money demand and therefore r?
• Closed economy: did I link r to private investment? Open economy: did I link r to capital flows, the exchange rate, and net exports?
• Did I re-read the question for special assumptions such as a liquidity trap, a fixed exchange rate, or an accommodating central bank?

This guide was prepared by CampusDown Editorial drawing on John R. Hicks, "Mr. Keynes and the 'Classics'" (1937); John Maynard Keynes, The General Theory of Employment, Interest and Money (1936); the standard intermediate texts by N. Gregory Mankiw and Olivier Blanchard; and publicly reported Federal Reserve, Bank of England, and Congressional Budget Office data. Reviewed November 2026. Because IS-LM-BP assumes sticky prices in the short run, pair it with an aggregate supply framework or the IS-MP model when analyzing inflation episodes such as 2021–2023.