IS Curve

Background

The IS curve stands for “Investment-Savings” curve, is a crucial concept in macroeconomic analysis, particularly within the IS-LM (Investment-Saving / Liquidity preference-Money supply) model of Keynesian economics. It represents the relationship between aggregate output (or national income) and interest rates, where the goods market is in equilibrium — that is, where planned (or ex ante) savings equal planned investment.

Historical Context

Introduced by John R. Hicks in 1937 using Keynes’s ideas from “The General Theory of Employment, Interest and Money” (1936), the IS curve aims to illustrate the market for goods and services. It has since become a fundamental element in macroeconomics for analyzing fluctuations in national income and policy impacts.

Definitions and Concepts

Ex ante savings: Planned or intended savings by households at a given level of national income and interest rate.
Ex ante investment: Planned or intended investments by firms at a given level of interest rate.
Product market equilibrium: A state where planned expenditures on goods and services by households and firms align, meaning ex ante savings equal ex ante investment.

Major Analytical Frameworks

Classical Economics

Classical economics does not explicitly use the IS curve as the primary focus for classic approaches centered around full-employment equilibrium and market-clearing models absent of containing a properly defined IS curve.

Neoclassical Economics

While neoclassical economics builds on classical tenets emphasizing market efficiency and full-employment equilibrium, it integrates the IS curve’s notion of market-driven adjustments between savings and investment.

Keynesian Economics

In Keynesian economics, the IS curve is indispensable. Keynes posited that product market equilibrium is subject to failures leading to cyclical developments and that interest rates and national income are pivotal:

When national income (Y) increases, savings (S) tend to rise significantly.
Modifications in the interest rate (r) have profound effects on investment (I).
Higher interest rates reduce investment sharply whereas savings change little.

These concepts determine the negative slope of the IS curve in an r-Y (interest rate-national income) plot, reflecting that increases in Y necessitate decreases in r to maintain equilibrium.

Marxian Economics

Marxian analysis doesn’t explicitly emphasize the IS curve but rather focuses on class relations, modes of production, and capital accumulation dynamics affecting savings and investment indirectly.

Institutional Economics

Institutional economics might regard the IS curve regarding the impacts of institutions, regulations, and norms on saving behavior and investment patterns affecting economic equilibrium.

Behavioral Economics

Behavioral economics integrates factors such as biases, heuristics, and irrational behaviors that might influence savings and investments, potentially distorting the anticipated equilibrium portrayed by the IS curve.

Post-Keynesian Economics

Post-Keynesians delve further into uncertainties and suggest alterations to the conventional IS curve analysis, reflecting more complex dynamics and possible non-linear relationships between savings, investment, and income.

Austrian Economics

Austrian economics, which prioritizes time preferences and capital goods structures, would critique the IS curve for oversimplifying intertemporal economic relations.

Development Economics

Development economics could employ the IS curve to analyze the impacts of varying interest rates and income levels in different stages of economic development and across various institutional settings.

Monetarism

Monetarists, advocating stern control of money supply instead of focusing directly on IS curve relationships, might still use it to understand dynamics underlying fiscal policy changes.

Comparative Analysis

IS Curve and LM Curve

The LM curve (Liquidity preference-Money supply) complements the IS curve within the IS-LM model by depicting equilibrium in the money market. Their intersection defines the general equilibrium levels of interest rates and output in the economy.

IS Curve Across Economics Schools

While designed primarily within Keynesian frameworks, the IS curve conceptually adapts across different economic schools reflecting their divergent theories about savings, investments, interest rates, and income equilibrium.

Case Studies

Post-2008 Financial Crisis Analysis: Analyzing changes in the IS curve helping understand the impact of fiscal stimulus measures on national income and interest rates dynamics.
The 1970s Stagflation: Examine shifts in the IS curve with elevated inflation and unemployment, scrutinizing effectiveness of interest rate adjustments.

Suggested Books for Further Studies

“Macroeconomics” by N. Gregory Mankiw
“Advanced Macroeconomics” by David Romer
“The General Theory of Employment, Interest, and Money” by John Maynard Keynes
“Monetary Theory and Policy” by Carl E. Walsh

**LM