Keynes and the Birth of the AD–AS Model

The 総合供給と総合需要モデル, also called the AD–AS model, is a foundational framework in modern macroeconomics. The model’s roots trace back to John Maynard Keynes and his book, "The General Theory of Employment, Interest, and Money." Published in 1936, this work marked a turning point in economic thought and introduced concepts that form the basis of this model.

Aggregate demand, or AD, in the model represents the total demand for goods and services in an economy at a given overall price level and in a given period. This demand includes consumption by households, investment by businesses, government spending, and net exports, which is the value of exports minus the value of imports. The AD curve is typically downward sloping, reflecting the inverse relationship between the price level and the quantity of output demanded. As prices rise, purchasing power falls, which tends to reduce overall demand.

Aggregate supply, or AS, represents the total quantity of goods and services producers in an economy are willing to supply at various price levels, over a specific period. In the short run, the AS curve is usually upward sloping, which means that as the price level increases, producers are willing to supply more goods and services. This happens because higher prices often mean higher revenues and profits for businesses, incentivizing them to increase production.

The intersection of the AD and AS curves determines an economy’s equilibrium price level and output. At this intersection point, the quantity of goods and services demanded equals the quantity supplied. If the economy is not at this equilibrium, there will be either surplus or shortage in the market, which causes price and output to adjust until equilibrium is restored.

Keynes’s insights were incorporated into the AD–AS model, making it one of the most widely used tools for analyzing macroeconomic fluctuations such as recessions and inflation. The model is a key part of the Keynesian school of thought, but it is also employed by economists from other schools, such as those following monetarist and classical approaches.

The AD–AS model helps explain how changes in aggregate demand or aggregate supply can cause shifts in output and price levels. For example, if government spending increases, aggregate demand will shift to the right—this means, at every price level, more goods and services are demanded. The immediate effect is an increase in both output and the price level, resulting in what is commonly referred to as demand-pull inflation.

On the supply side, if factors like technological improvement or increased availability of raw materials reduce production costs, the aggregate supply curve can shift right.

The original AD–AS model’s development was deeply influenced by the economic hardships of the 1930s. John Maynard Keynes introduced the idea that total demand, not just supply, could limit an economy’s ability to reach full employment. Before Keynes, many economists believed that markets would always clear, and that any unemployment would be temporary. Keynes argued that insufficient demand could lead to prolonged economic slumps, which is why the AD–AS model emphasizes the role of demand shocks, such as changes in investment or government spending, in driving economic cycles.

Over time, the AD–AS model has served as a reference for many schools of thought, including monetarists who emphasize the role of money supply and price mechanisms in the economy. The model is also a foundational concept in modern macroeconomic textbooks, including "Economics: An Introduction" by Chris Mulhearn, Howard R. Vane, and James Eden, published in 2005 by 五南圖書出版股份有限公司 in Taipei. This textbook, identified by ISBN 978-957-11-4171-8, dedicates significant discussion to the model’s mechanics and applications.

Another major publication that discusses the AD–AS model is 陳超塵’s "Principles of Economics," published by 臺灣商務印書館 in 1996, with ISBN 978-957-05-1248-9. This work explores the model’s relevance in understanding inflation, unemployment, and economic policy.

The AD–AS model graphically is comprised of two curves—aggregate demand sloping downward and aggregate supply sloping upward, intersecting at equilibrium. If, for instance, a government implements a fiscal stimulus package, the aggregate demand curve shifts right. This creates upward pressure on both output and the price level. The scale of these effects depends on the slope of the aggregate supply curve: if the AS curve is relatively flat, increases in demand mainly boost output; if it’s steep, increases in demand mostly raise prices.

Alternatively, imagine a supply shock such as sudden increases in oil prices. This raises production costs across the economy and shifts the aggregate supply curve leftward. The immediate result is lower output and higher prices, a phenomenon known as stagflation.

The AD–AS model is versatile. It can accommodate different time frames. In the short run, wages and prices may be sticky—meaning they don’t adjust instantly to changes in demand or supply. This rigidity is one reason the short-run aggregate supply curve is not vertical. Over a longer time frame, as contracts are renegotiated and expectations change, the aggregate supply curve becomes steeper and may even become vertical at full-employment output, reflecting the economy’s potential output or natural level of production.

Keynes’s foundational ideas in the AD–AS model were initially met with skepticism by proponents of classical economics, who favored self-correcting markets. However, the model’s ability to explain persistent unemployment during the Great Depression and its usefulness in analyzing the effects of fiscal and monetary policy led to its widespread adoption among macroeconomists.

The model is used by central banks and policymakers to forecast the effects of policy changes. For example, if a central bank increases the money supply, this typically lowers interest rates, stimulating investment and consumption, and shifting the aggregate demand curve right. Policymakers use the model to weigh potential inflationary pressures against the need to stimulate economic growth.

Despite its utility, the AD–AS model has limitations. It simplifies many complex processes and assumes that all prices adjust in response to shifts in demand and supply. In reality, prices and wages can be slow to respond, and factors like expectations, market imperfections, and global economic conditions can complicate the model’s predictions.

It appears as a key topic in university-level courses and is referenced in exam syllabi for economics students.

John Maynard Keynes’s book, "The General Theory of Employment, Interest, and Money," published in 1936, remains one of the most cited works in the field of economics. It laid the groundwork for the analysis of business cycles and government intervention.

The AD–AS model is classified as both a macroeconomics model and a curve analysis tool, often grouped alongside the IS–LM model in academic literature.

The model has been used to explain major historical episodes, including the global economic downturns of the 20th century, by showing how shifts in aggregate demand or supply can result in recessions or inflation.