Krugman has an interesting article today, Monetary Policy in a Liquidity Trap. I (sort of) agree with much of it. But I believe that a few comments are in order.
Consider this statement:
So, at this point America and Japan (and core Europe) are all in liquidity traps: private demand is so weak that even at a zero short-term interest rate spending falls far short of what would be needed for full employment. And interest rates can’t go below zero (except trivially for very short periods), because investors always have the option of simply holding cash.
This statement is, in varying degrees:  interpretative,  assertive,  misleading, and  wrong.
First, the quoted passage above suggests that a liquidity trap is the byproduct of “insufficient private demand,” with the implication, of course, that more “public demand” is needed to rectify the situation. This may or may not be true. Regardless, the statement is , , and  above. Beware of economists making bald assertions.
Second, the statement is wrong in suggesting that our current liquidity trap is associated with zero nominal interest rates. Liquidity trap phenomena are much more general than this. And if you really want to further your understanding on this matter, please go read this piece by Steve Williamson: Liquidity Traps, Money, Inflation, and Bond Yields. As Steve says: this is not your grandma’s liquidity trap.
In grandma’s liquidity trap, the real interest rate is too high because of the zero lower bound. Steve argues that in our current liquidity trap, the real interest rate is too low, reflecting the huge world appetite for relatively safe assets like U.S. treasuries.
If this latter view is correct, then “corrective” measures like expanding G or increasing the inflation target are not addressing the fundamental economic problem: low real interest rates as the byproduct of real economic/political/financial factors.
Given these “real” problems, Steve’s view is that the Fed is largely irrelevant. But he does assign hope to the Treasury: increase the supply of its securities to meet the world demand for them. I’ve been making similar arguments for some time now; for example, here.
Apart from all this, it will be interesting to see how the experiment in Japan plays out. Most of the massive purchases announced by the BOJ are for JGBs — I’m really skeptical what sort of effect this should have (since the operation constitutes swaps of two assets that are close to perfect substitutes–although some purchases will take the form of higher risk assets–see Noah Smith on this). But what I think really does not matter–it is what market participants think–and the program does appear to be having some effect in financial markets.
Thank you, Japan, for this interesting experiment. Domo arigato, gozaimasu!