Explaining S=I: Inventories vs Adding up Individuals

Dec 05 13:12 2018 Print This Article

It's easy to teach students the arithmetic showing that actual saving must equal actual investment (S=I). But many students (quite rightly) want more than the arithmetic. Because S=I is not intuitive, and good students want to understand the intuition. I think that most profs will try to explain the intuition by talking about inventories of unsold goods. But I think it would be better to explain the intuition behind S=I by talking about the difference between individual saving and aggregate saving; some types of individual saving don't add up to aggregate saving.

A student's email is what prompted me to write this post. He's read three textbooks on the subject, and has read my old post, and understands the arithmetic, but still doesn't understand the intuition. Here's one of his questions: "Or, suppose I buy a used car. Is it saving? How will in this example saving be equal to investment?". Here's how I would answer his question: "Yes, your buying a used car is saving, because you spent part of your income on something that is not a newly-produced consumption good. Which is how we define "saving" in National Income Accounting. But someone else must have sold you that same used car, so if you were saving (buying an asset), that someone else was dissaving (selling an asset). It is impossible for everyone to save by buying used cars."

Read More

About Article Author

A Worthwhile Canadian Initiative

Group economics blog written by Canadian Economics professors and authors from the University of Laval, Carleton University, Richard Ivey School of Business, and Lakehead University

Related Items

US REITs Suffer Biggest Weekly Loss So Far In 2019

Real estate investment trusts (REITs) in the US tumbled last week, posting the biggest setback for the major asset classes during the shortened trading week that ended on April 18, based on a set of exchange traded funds. The loss, the deepest for this corner of the market since a plunge late last ...

Surge in Oil Punctures Holiday Markets

Overview: With many centers closed for the extended holiday, the calm in the global capital markets has been punctuated by reports that the US is considering ending its exemption for eight countries to have bought Iranian oil over the past six months.  The waivers were to end on May 2, but previ ...

EUR/USD – Euro yawing on Easter Monday, but U.S oil sanctions could rattle markets

German banks are closed for the Easter Monday holiday and EUR/USD is almost unchanged. Currently, the pair is trading at 1.1249, up 0.03% on the day. There are no German or eurozone events on the calendar. In the U.S., there is just one release. Existing home sales is expected to slow to 5.31 mill ...

Building a Sustainable Workforce with the integration of AI Solutions

Canada has emerged as a leader in the global race to develop artificial intelligence(AI) and we need to ensure the continued freedom to operate and our country’s business competitiveness. Through a series of roundtables this Spring, we are identifying positive outcomes for AI, exploring the range ...

Robinson Crusoe and the Carbon Tax

Suppose I impose a carbon tax on Robinson Crusoe. But I give him a rebate exactly equal to the tax he pays. That tax plus rebate will have no effect on Robinson Crusoe's behaviour. He knows that if he cuts carbon by 1kg, and pays $1 less tax, his rebate also falls by $1, so his net tax (= tax minus ...

Daily Markets Broadcast 2019-04-22

Daily Markets Broadcast 2019-04-22 Wall Street extends gains on strong retail sales US indices continued to rally on Thursday after retail sales data for March came in higher than expected. The US30 index has continued the rally in early trading this morning. Markets were closed on Friday and many ...