クルーグマンも貨幣乗数を否定

セントルイス連邦準備銀行が金融システムの仕組みに関する理解のアップデートの必要性について書いていたが、

Many econ textbooks include outdated information on how Fed policy influences banks and the economy. Educators should abandon the “money multiplier,” a popular model that is now obsolete, a Page One Economics article says https://t.co/0yW8iAxDLe pic.twitter.com/7RXHg474LZ

— St. Louis Fed (@stlouisfed) October 11, 2021

貨幣乗数の無意味さ（→捨て去るべし）について、クルーグマンも同じ理解に到達していた。量的緩和が高インフレを招かないことも、現実の通貨システムの仕組みを知っていれば自明だった。

Good to see Cathie Wood taking on yet another tech bro who for some reason thinks he understands monetary economics. But ... 1/ pic.twitter.com/f3bJWOWkVV

— Paul Krugman (@paulkrugman) October 26, 2021

Thinking of this in terms of the velocity of money is itself a misunderstanding. When you're in a liquidity trap, with short-term rates near zero, the quantity of money is both endogenous and irrelevant 2/

— Paul Krugman (@paulkrugman) October 26, 2021

Some guy wrote about this a while back 3/ https://t.co/ZEILV7tmbT

— Paul Krugman (@paulkrugman) October 26, 2021

Friends tell me that this tweet was obscure — and it seems that many people, even in the finance world, don't get why velocity is unhelpful now. So, a thread 1/ https://t.co/Fm0nbx7v2I

— Paul Krugman (@paulkrugman) October 26, 2021

Start with what happened in the first few years of the financial crisis and aftermath. Here's the monetary base, which is what the Fed controls directly, one measure of the money supply, and nominal GDP 2/ pic.twitter.com/6FUon6sHJG

— Paul Krugman (@paulkrugman) October 26, 2021

Obviously monetary base (M0) grew enormously, M2 some but not much, GDP even less. So as a matter of arithmetic velocity of either M0 or M2 fell. But why? Because M0 was in a fundamental sense disconnected from GDP 3/

— Paul Krugman (@paulkrugman) October 26, 2021

Normally an open market operation, in which the Fed increases the monetary base by buying Treasuries, has major effects because M0 is the medium of exchange, which means that people hold it despite a zero interest rate for the sake of liquidity 4/

— Paul Krugman (@paulkrugman) October 26, 2021

So increasing M0 increases liquidity, which sort of rolls through the economy via lower interest rates. But from late 2008 on the interest rate on short-term Treasuries was essentially zero, which meant that holding monetary base was costless 5/

— Paul Krugman (@paulkrugman) October 26, 2021

And that in turn meant that at the margin M0 and Treasuries were perfect substitutes — two assets with the same (zero) rate of return. Conventional open-market operations therefore did nothing — investors were already saturated with liquidity, increasing M0 added nothing 6/

— Paul Krugman (@paulkrugman) October 26, 2021

That's why we call it a liquidity trap. Normal monetary policy becomes irrelevant. Huge increases in M0 had basically no effect on GDP. I'll get into slight complications in a second, but that's the key insight 7/

— Paul Krugman (@paulkrugman) October 26, 2021

And when M0 doesn't affect GDP, a rise in M0 will, as a matter of arithmetic, reduce the ratio of GDP to M0 — velocity. But it makes no sense to focus on that ratio 8/

— Paul Krugman (@paulkrugman) October 26, 2021

Think of two gears that normally mesh with each other, but are currently separated. If we spin gear A faster and nothing happens to gear B, would it make sense to say that the gearing ratio has declined? No, they're just spinning independently 9/

— Paul Krugman (@paulkrugman) October 26, 2021

OK, two complications. The Fed engaged in unconventional open market operations, buying longer-term bonds etc — QE. These may have had some effect. But if so, it wasn't through anything like the normal relationship btwn money supply and GDP 10/

— Paul Krugman (@paulkrugman) October 26, 2021

Also, people who talk about the money supply usually mean an aggregate like M1 or M2. Normally the Fed can target those aggregates via its control of M0. But in a liquidity trap those gears are *also* disconnected. Increase bank reserves and they just sit there 11/

— Paul Krugman (@paulkrugman) October 26, 2021

So M1 and M2 are no longer policy variables, even potentially; they're determined mainly by demand for deposits, and the Fed can't change that through its operations. As I said, money is endogenous 12/

— Paul Krugman (@paulkrugman) October 26, 2021

Again, in an arithmetic sense the money multiplier has declined, but that's not a helpful way to describe what's going on, which is instead that the gears have simply been disengaged 13/

— Paul Krugman (@paulkrugman) October 26, 2021

So in a zero-rate environment, velocity and similar concepts just don't help you understand what's happening. If anything, they create confusion by suggesting that the size of the monetary base matters under conditions where it doesn't 14/

— Paul Krugman (@paulkrugman) October 26, 2021

Again, I went through all this in 1998 (and James Tobin could have told you most of it decades earlier) 15/ https://t.co/ZEILV7tmbT

— Paul Krugman (@paulkrugman) October 26, 2021

現実にはあり得ないことではあるが、中央銀行の決済システムがダウンして銀行間決済ができなくなり、市中での現金決済オンリーになったとする。民間銀行は現金需要→預金引き出しの増加に備えて中銀当座預金を現金化し、店舗やATMの「現金在庫」を増やす。

量的緩和とは「現金在庫」が必要以上に溢れかえるように中央銀行が民間銀行に現金を供給することだが、それが民間部門の銀行借入や名目GDPの拡大に直結しないことは明らかだろう。