But even in the not-as-bad case, in which prices only fall a little and wages don't, it can still maim the economy. People put off purchases and companies put off investments when things cost more today than tomorrow. And over-leveraged households and businesses have a hard time cutting their debts to more manageable levels when inflation isn't helping them out. That's true even if inflation is a low positive, instead of a low negative, number—what the IMF fittingly calls "lowflation."

Irresponsible responsibility

So why did Japan let itself get stuck in this deflationary trap? Let prices fall more than real GDP grew, so that its total economy shrank? Well, it was afraid to do what it needed to do to get out. The Bank of Japan (BOJ) couldn't just cut interest rates like it normally would, because rates were already at zero. So it had to try new things, unconventional things—and it didn't want to. Instead, it said that there was nothing it could do. Or that a little deflation was actually a good thing.

Now, it wouldn't have been easy for Japan to shake its post-bust blues. But it didn't even try. As Paul Krugman pointed out in 1998, the problem was that the BOJ couldn't cut interest rates to increase inflation today, so it needed to "credibly promise to be irresponsible" to increase expected inflation tomorrow. In other words, it needed to convince markets that it was going to keep rates lower than it "should" once the recovery got going. But that was too much for a staid central bank like the BOJ. It was more worried about inflation than a lost decade. So it very responsibly did nothing. It was, as another Princeton professor named Ben Bernanke put it in 1999, "a case of self-induced paralysis."

Rooseveltian resolve ...

It only took 15 years, but the BOJ is finally waking up. Japan's prime minister, Shinzo Abe, has made whipping deflation a priority—and he's stacked the central bank with people who agree. In the past year, the BOJ has said it will do "whatever it takes" to stop prices from falling, and it's backed that up. It's announced a two percent inflation target, and said that it will keep buying long-term bonds until inflation gets and stays there.

It's working. The stock market has soared. Core inflation is at a 15-year high. And workers are starting to demand, and get, raises. But this might not be enough. A new Brookings paper estimates that this added about one percentage point to 2013 growth—a good start, but just that. Japan is still far from where it should be, and far from growing fast enough to get there. The question is if this is as good as it gets. And the answer depends on how you think bond-buying works.

"The problem with quantitative easing (QE)," Bernanke has said, "is it works in practice, but it doesn't work in theory." Now, there are two basic stories about why it does actually work. The first is mechanistic: When the Fed buys Treasury or mortgage bonds, the banks that owned them will, hopefully, buy other things. That might be stocks or corporate bonds or something else entirely. Whatever the case, it should push those asset prices up. And that should help companies borrow for less and make the people who own these things feel richer—and more likely to spend. If this is how QE works, then the BOJ can do a little bit better, but not much more, by buying different types of assets than it already is.