Poor Ben Bernanke!

As Chairman of the United States Federal Reserve Board, he has gone further than any other central banker in recent times in attempting to stimulate the economy through monetary policy.

He has cut short-term interest rates to the bone. He has adopted innovative new methods of monetary easing. Again and again, he has repeated that, so long as inflationary pressure remains contained, his main concern is the high level of US unemployment.

Yet progressive economists chastise him for not doing enough.

What more could they possibly want? Raise the inflation target, they say, and all will be well.

Of course, this would be a radical departure for the Fed, which has worked hard to convince the public that it will keep inflation around 2 percent.

Aggressive Fed

That credibility has allowed the Fed to be aggressive: it is difficult to imagine that it could have expanded its balance sheet to the extent that it has if the public thought that it could not be trusted on inflation.

So why do these economists want the Fed to sacrifice its hard-won gains?

The answer lies in their view of the root cause of continued high unemployment: excessively high real interest rates. Their logic is simple.

Before the financial crisis erupted in 2008, consumers buoyed US demand by borrowing heavily against their rising house prices.

Now these heavily indebted households cannot borrow and spend any more. An important source of aggregate demand has evaporated.

As consumers stopped buying, real (inflation-adjusted) interest rates should have fallen to encourage thrifty households to spend.

But real interest rates did not fall enough, because nominal interest rates cannot go below zero.

By increasing inflation, the Fed would turn real interest rates seriously negative, thereby coercing thrifty households into spending instead of saving. With rising demand, firms would hire, and all would be well.

This is a different logic from the one that calls for inflation as a way of reducing long-term debt (at the expense of investors), but it has equally serious weaknesses.

Traditional savers

First, while low rates might encourage spending if credit were easy, it is not at all clear that traditional savers today would go out and spend.

Think of the soon-to-retire office worker. She saved because she wanted enough money to retire.

Given the terrible returns on savings since 2007, the prospect of continuing low interest rates might make her put even more money aside.

Alternatively, low interest rates could push her (or her pension fund) to buy risky long-maturity bonds. Given that these bonds are already aggressively priced, such a move might thus set her up for a fall when interest rates eventually rise.

Indeed, America may well be in the process of adding a pension crisis to the unemployment problem.

Second, household over-indebtedness in the US, as well as the fall in demand, is localized, as my colleague Amir Sufi and his co-author, Atif Mian, have shown.

Hairdressers in Las Vegas lost their jobs partly because households there have too much debt stemming from the housing boom, and partly because many local construction workers and real-estate brokers were laid off. Even if we can coerce traditional debt-free savers to spend, it is unlikely that there are enough of them in Las Vegas.

If these debt-free savers are in New York City, which did not experience as much of a boom and a bust, cutting real interest rates will encourage spending on haircuts in New York City, which already has plenty of demand, but not in Las Vegas, which has too little.

Put differently, real interest rates are too blunt a stimulus tool, even if they work.

Third, we have little idea about how the public forms expectations about the central bank's future actions.

If the Fed announces that it will tolerate 4 percent inflation, could the public think that the Fed is bluffing, or that, if an implicit inflation target can be broken once, it can be broken again?

Would expectations shift to a much higher inflation rate?

How would the added risk premium affect long-term interest rates?

What kind of recession would the US have to endure to bring inflation back to comfortable levels?

The answer to all of these questions is: We really don't know.

Given the dubious benefits of still lower real interest rates, placing central-bank credibility at risk would be irresponsible.

Finally, it is not even clear that the zero lower bound is primarily responsible for high US unemployment.

Traditional Keynesian frictions like the difficulty of reducing wages and benefits in some industries, as well as non-traditional frictions like the difficulty of moving when one cannot sell (or buy) a house, may share blame.

High unemploymet

We cannot ignore high unemployment. Clearly, improving indebted households' ability to refinance at low cur-rent interest rates could help to reduce their debt burden, as would writing off some mortgage debt in cases where falling house prices have left borrowers deep underwater (that is, the outstanding mortgage exceeds the house's value).

More could be done here.

The good news is that household debt is coming down through a combination of re-payments and write-offs.

But it is also important to recognize that the path to a sustainable recovery does not lie in restoring irresponsible and unaffordable pre-crisis spending, which had the collateral effect of creating un-sustainable jobs in construction and finance.

With a savings rate of barely 4 percent of GDP, the average US household is unlikely to be over-saving.