If the Fed was so aggressive, why didn't we have inflation?

 In a recent podcast with Robert Hall, Russ Roberts asks:

"If the Fed was so aggressive, why didn't we have inflation? And does that mean that Milton Friedman and others were wrong?"

It's a good question. Because I find monetary policy confusing, I want to try answering Russ's question with an analogy to an example that doesn't involve money.

Say there are two types of gold rings, those with diamonds and those without. The price of gold rings with diamonds exceeds the price of rings without diamonds by a wedge that equals the price of the diamond.

A technology emerges that can create diamonds at zero cost. The supply of diamonds will rapidly grow until they become like water; while boasting desirable qualities, a diamond will sell for $0. When this happens the price of gold rings with diamonds will equal the price of gold rings without.

Using this analogy, we can understand why—despite having been so aggressive—the Fed didn't create inflation. Treasury debt and Fed debt are alike in that they are both government liabilities. However, Fed debt comes with an extra cherry on top; it can be spent anywhere. Government debt... not so liquid. This mobility is a valuable commodity and people will (typically) pay a premium to own it. So we might say that Treasury debt is very much like our plain gold band, and Fed debt is like a gold band with a diamond attached to it.

When the Fed expands aggressively, it does so through open market operations, or by spending its own Fed debt to acquire Treasury debt. What effect do these operations have?

Let's look at how open market operations would work in the ring market. A ring producer that has developed a technology to create diamonds at zero cost begins to "spend" new gold bands (with diamonds) into the economy by purchasing gold bands (without diamonds). The number of gold bands in the economy will stay constant (x gold is being swapped for x gold). But the quantity of diamonds in the economy increases. On the margin, diamonds are becoming less valuable, and so the price of gold bands with diamonds falls. We get inflation in the price of gold bands with diamonds.

However, this inflation will eventually come to a stop. Once the price of diamonds has fallen to its lower bound of zero, the price of rings with diamonds will equal the price of a rings without. Subsequent spending by our ring producer of new gold bands with diamonds into the economy will have no effect—all that is happening is a swap of a gold band for a gold band, and a swap of like-for- like has no macroeconomic effect.

And that's why the Fed's aggressiveness (QEI-III) has had little effect on prices. Once the Fed has conducted enough open market operations, the useful commodity attached to Fed debt that we call mobility—much like the diamond in our previous example—becomes so prevalent that on the margin it is worth zero. At this point, Fed debt loses its uniqueness and is exactly the same as Treasury debt. All subsequent purchases are irrelevant because the Fed is simply switching like-for-like. Thus the Fed, like our ring producer, has lost the ability to create inflation via open market operations.