Simon Wren-Lewis ved Oxford University forsøker å beskrive alternativer til inflasjonsmålet, nemlig produksjonsmål og hvordan dette vil skje når renten er tilnærmet null. Dette går inn i diskusjonen om hvorvidt sentralbanker vurderer inflasjon viktigere enn arbeidsledighet i rentesettingen, og at et inflasjonsmål er feil medisin i resesjoner.
Første gang han forsøkte å forklare dette var det algebra invlolvert. Vi vet alle hvor pedagogisk det er. Så her er han uten:
Output today depends on real interest rates today. Anything else that changes output today is called a shock. There is no Quantitative Easing (QE). For simplicity we will assume no direct linkage between output across periods. Inflation today depends on output today, but also expected inflation tomorrow. Prices were on target yesterday, and let’s assume that inflation the day after tomorrow is fixed. Now hit output today with a negative demand shock, a shock which is not repeated tomorrow. If that shock is small enough, we can offset it entirely by lowering interest rates today. Inflation today is unchanged as a result. Nothing changes tomorrow. Everyone is happy: monetary policy has done its job, by lowering nominal and real interest rates today.
Now suppose the negative shock today is so large, that even with nominal interest rates at zero, output remains too low. This reduces inflation today. With a target for NGDP, not only would this mean we missed our target today, but it would also mean that we would miss our NGDP target tomorrow, because lower inflation today would mean lower prices tomorrow if inflation tomorrow was unchanged. So monetary policy cuts interest rates tomorrow in order to raise tomorrow’s output and inflation. If this response is expected (we assume rational expectations throughout) it does two things. It raises inflation today directly (because inflation today depends on expected inflation tomorrow), and it reduces real interest rates today (real interest rates today are nominal rates today minus expected inflation tomorrow), which raises output today which in turn raises inflation today. This is why NGDP targets can be very helpful to combat the ZLB. If we instead targeted inflation, then monetary policy tomorrow might do nothing, because policy would still hit the inflation target tomorrow.
Now suppose that, despite relaxing monetary policy tomorrow, we still hit the ZLB today. Output and inflation are too low today, even though we hit the NGDP target tomorrow. There is absolutely no reason why this might not happen. Although inflation is higher tomorrow to compensate for low inflation today, real interest rates today are still not low enough to prevent output falling today. Sure, we could relax monetary policy tomorrow by yet more (assuming we are not at the ZLB tomorrow), and this could increase inflation tomorrow by enough to get real interest rates low enough today. But that would mean we overshoot our NGDP target tomorrow.
So, we hit the ZLB today, undershoot the NGDP target today, but meet it tomorrow. Now let output also depend on fiscal policy, and add some austerity today. This reduces output by yet more today. Interest rates are at the ZLB today, so monetary policy today can do nothing about it. Lower output lowers inflation today still further, but because this reduces the price level tomorrow, monetary policy relaxes by more tomorrow. This is good, because higher inflation tomorrow raises both inflation and output today, which moderates the impact of austerity. But the end result must be that the net impact of austerity is to reduce output and inflation today, and raise inflation tomorrow. (If output and inflation are not lower today, inflation will not change tomorrow. But if inflation tomorrow is unchanged, why has austerity not reduced output and inflation today?)
That is how a negative demand shock today, like austerity, can raise inflation tomorrow. It only happens because we have a target for the level of NGDP, and we cannot use interest rates to kill the impact of the demand shock today, because we are at the ZLB. But the latter is where we currently are, if you ignore QE.
It follows directly that excess austerity today reduces welfare, even with NGDP targets. We still hit the NGDP target tomorrow, but only by having additional excess output and inflation tomorrow, and a bigger recession today. People are clearly worse off both today and tomorrow.
This little experiment also shows us why fiscal stimulus can be more effective than a NGDP target at stabilizing the economy at a ZLB. Just put the story in reverse. Instead of excess austerity, now have a fiscal stimulus today sufficient to get NGDP back to target today. As there is no shock tomorrow (fiscal or otherwise), we also hit NGDP tomorrow. We are back to where we want to be. Now the fiscal stimulus may have meant more spending than we might have wanted in the short term, so it’s not cost free. But in terms of output and inflation, we are clearly better off as a result of the fiscal stimulus, even though we have a NGDP target.