AP Macroeconomics Unit 3 Essentials

Unlike a simple supply-and-demand graph for one good, the AD/AS model determines two macroeconomic variables at once — the overall price level and real GDP. Every shift in either curve moves both variables, sometimes in the same direction and sometimes in opposite directions. Reading which curve shifted, and predicting both the price level and output effects, is the single most-tested skill in this unit.

When the government spends an extra dollar, that dollar becomes someone's income, who then spends part of it (their MPC), which becomes someone else's income, and so on. This ripple effect — the multiplier — means a relatively small initial change in spending can produce a much larger total change in GDP. Understanding the multiplier is essential to predicting how powerful a fiscal policy action will actually be.

If left alone, wage and price flexibility will eventually push the economy back to potential GDP after a shock — SRAS shifts until equilibrium meets LRAS. But this process can take years, and in the meantime, a recessionary gap means real unemployment and lost output for real people. That gap between "the economy will fix itself eventually" and "people are suffering right now" is exactly why governments often choose to intervene with policy rather than wait.

Expansionary fiscal policy can close a recessionary gap, but increased government borrowing can raise interest rates and crowd out private investment, partially blunting the policy's own effect. Contractionary fiscal policy can cool inflation, but at the cost of slower growth and higher unemployment in the short run. No fiscal tool is free — every choice to use one comes with a cost that shows up somewhere else in the model.