In a 1963 paper Robert Mundell first argued that higher inflation had real effects. He challenged the classical dichotomy by suggesting that nominal interest rates would rise less than proportionally with inflation, because higher price levels would induce a fall in money demand, thereby increasing velocity and capital formation which, in turn, would bring real rates down. The most interesting part of my argument comes from a model designed by Eric Kam in 2000, which I’ll get to.

And as Japan emerges from a liquidity trap, the Mundell-Tobin effect (named, too, for James Tobin submitting a similar mechanism) should anchor our intellectual framework. I don’t see any of the best bloggers (I may be wrong but see the self-deprecation there via Google) arguing along these lines, though Krugman offers a more sophisticated explanation of the same thing through his 1998 model, this can only strengthen our priors.

Paul Krugman, Noah Smith, Brad DeLong, and Nick Rowe have each replied to a confused suggestion from Richard Koo about monetary stimulus. Smith, as Krugman points out, was restricting his analysis on purely nominal scope and notes that DeLong captures the risk better, so here’s DeLong:

But if Abenonomics turns that medium-run from a Keynesian unemployment régime in which r &ltl g to a classical full-employment régime in which r > g, Japan might suddenly switch to a fiscal-dominance high-inflation régime in which today’s real value of JGB is an unsustainable burden..

Moreover, to the extent Abenomics succeeds in boosting the economy’s risk tolerance, the wedge between the private and public real interest rates will fall. Thus Paul might be completely correct in his belief that Abenomics will lower the real interest rate–but which real interest rate? The real interest rate it lowers might be the private rate, and that could be accompanied by a collapse in spreads that would raise the JGB interest rate and make the debt unsustainable.

I’ll address the latter concern first. Let’s consider the premise “to the extent Abenomics succeeds in boosting the economy’s risk tolerance”. If the whole scare is about Japan’s ridiculously-high debt burden, and we’re talking about the cost of servicing that debt, as far as investors are concerned isn’t Japan’s solvency a “risk”. I don’t think it is, I certainly don’t see a sovereign default from Japan, but that’s the presumed premise DeLong sets out to answer. So with that clause, the question becomes self-defeating, as increased risk tolerance would convince investors to lend Japan more money. Note the implicit assumption I’m making here is that it’s possible for a sovereign currency to default. I make this because there are many cases where restructuring (“default”) would be preferable to hyperinflation.

Even ignoring the above caveat, the fall in interest rate spreads can come from both private and public yields falling, with the former falling more rapidly. A lot of things “might be”, and do we have any reason to believe it “might be” that inflation does nothing to real public yields?

Well, as it turns out, we have good reason beyond Krugman’s model suggesting that inflation increases only nominal yields:

• Mundell-Tobin argue that the opportunity cost of holding money increases with inflation, resulting in capital creation and decreased real rates. This is a simple explanation as any, but would be rejected as its a “descriptive” (read: non-DSGE) model.
• So comes along Eric Kam arguing that: The Mundell-Tobin effect, which describes the causality underlying monetary non-superneutrality, has previously been demonstrated only in descriptive, non-optimizing models (Begg 1980) or representative agent models based on unpalatable assumptions (Uzawa 1968). This paper provides a restatement of the Mundell-Tobin effect in an optimizing model where the rate of time preference is an increasing function of real financial assets. The critical outcome is that monetary superneutrality is not the inevitable result of optimizing agent models. Rather, it results from the assumption of exogenous time preference. Endogenous time preference generates monetary non-superneutrality, where the real interest rate is a decreasing function of monetary growth and can be targeted as a policy tool by the central monetary authority.

[Caution, be-warned that we can probably create a DSGE for anything under the sun, but I will go through the caveats here as well] Note that he’s not making any (further) remarkable constraints to prove his point, just relaxing a previous assumption that time preference is exogenous. A previous paper (Uzawa, 1968) followed a similar procedure, but made the strong, questionable, and unintuitive assumption that the “rate of time preference is an increasing function of instantaneous utility and consumption” which implies that savings are a positive function on wealth, which contradicts the Mundell-Tobin logic.

Kam, rather, endogenizes time preference as a positive function on real financial balances (capital + real balances). He shows non-neutrality with the more intuitive idea that savings are a negative function on wealth. (So unanticipated inflation would result in higher steady-state levels of capital).

Look, in the long-run even Keynesians like Krugman believe in money neutrality. By then, however, inflation should have sufficiently eroded Japan’s debt burden. DeLong’s worry about superneutrality in the medium-term, where debt levels are still elevated, seems unlikely even without purely Keynesian conditions. That is, no further assumptions other than an endogenous time preference are required to move from neutrality to non-neutrality. DSGEs are fishy creatures, but here’s why this confirms my prior vis-a-vis Abenomics:

1. Lets say superneutrality is certain under exogenous time preference (like Samuelson’s discounted utility).
2. Non-superneutrality is possible under endogenous time preference. Personally, I find a god-set discount rate/time preference rather crazy. You can assign a Bayesian prior to both possibilities in this scenario. But note, the models which support neutrality make many more assumptions.

Now we need a Bayesian prior for (1) vs. (2). My p for (1) is low for the following reasons:

• It just seems darned crazy.
• Gary Becker and Casey Mulligan (1997) – ungated here – quite convincingly discuss how “wealth, mortality, addictions, uncertainty, and other variables affect the degree of time preference”.

I’d add a few other points to DeLong’s comment – “the real interest rate it lowers might be the private rate, and that could be accompanied by a collapse in spreads that would raise the JGB interest rate and make the debt unsustainable” – if the idea of falling real yields is not plausible. A fall in private cost of capital should be associated with a significant increase in wages, capital incomes, and at least profits. This by itself increases more revenue, but likely a greater portion of the earned income will fall in the higher tax brackets, suggesting a more sustainable debt. Of course my belief is that both private and public debt will be eroded quicker, but even if that’s too strong an assumption, there’s no reason to believe that falling spreads per se are a bad thing for government debt so long as it maintains tax authority.

However, the Mundell-Tobin and similar effects derive from a one-time increase in anticipated inflation. It’s not even to be seen that Japan will achieve 2%, and that’s a problem of “too little” Abenomics. On the other hand, if Japan achieves 2%, and tries to erode even more debt by moving to 4% it will loose credibility. Therefore, Japan should – as soon as possible – commit to either a 6% nominal growth target or 4% inflation target.

This is preferable because it increases the oomph of the initial boost, but primarily it extends the duration of the short run where monetary base is expanding and inflation expectations are rising. A longer short-run, we minimizes DeLong’s tail risks of a debt-saddled long-run, even if you reject all the above logic to the contrary.

DeLong concludes:

Do I think that these are worries that should keep Japan from undertaking Abenomics? I say: Clearly and definitely not. Do I think that these are things that we should worry about and keep a weather eye out as we watch for them? I say: Clearly and definitely yes. Do I think these are things that might actually happen? I say: maybe.