Lars Christensen has a post in which he uses an Aggregate Supply
/ Aggregate Demand (AS/AD) diagram to talk about monetary offset in the eurozone. This is the idea that if the central bank is pursuing an inflation target, then fiscal policy can have no effect because it will be automatically be countered by monetary policy. A good description of monetary offset is here.
This reminded me of one of the things that puzzled me when I
first learned about AS/AD and its interaction with IS/LM.
In the standard IS/LM model, the IS curve and the LM curve
each describe a different relationship between the interest rate and real GDP.
Each assumes that certain other variables are fixed, including the general
price level. Thus for any given price level, we can deduce a level of
real GDP from where the IS curve meets the LM curve. This also determines
the interest rate.
The relationship between the price level and real GDP gives
us our aggregate demand curve. This is usually assumed to be downward
sloping, so that as the price level falls, GDP rises (although it has been
argued that it might be upwards sloping in certain circumstances).
To determine where we are on the aggregate demand curve, we
combine it with an aggregate supply curve. Here, the convention is to
draw an aggregate supply curve that slopes upwards in the short run, but is
vertical in the long run. The idea is that price stickiness in the short
run limits price rises so that changes in demand create real changes in output,
but in the long run, once prices have fully adjusted, output must return to its
equilibrium level. (AS/AD is sometimes
done by references to changes, rather than absolute levels, i.e. as inflation against
real GDP growth).
So far, this is all pretty standard. There's plenty of
stuff there to question the validity of, but the issue I want to look at is to
do with the exchange rate.
The first thing to note is that the position of the short
run AS curve depends on the exchange rate. A depreciation of the exchange
rate will increase the effective price of foreign goods. This will tend
to increase the price of domestic goods that compete with foreign goods:
exports or domestic substitutes for imports. At the same time, higher
import costs will potentially reduce real wages. To the extent that there
is also real wage stickiness, nominal wages will rise by more than they
otherwise would. So, a depreciation will move the short run AS curve
upwards.
Again, this should not be controversial and I have see it discussed
before, but it's often not mentioned.
The interesting bit comes though when we try to combine this
with what is going on with IS/LM. The issue here is that one of factors
influencing the exchange rate is the interest rate. We already know that a
change in either the IS curve or the LM curve will shift the AD curve.
However, because it also has an impact on the interest rate we can now see that
it will also move the short run AS curve.
This seems to me to be fairly obvious. I learned this
stuff in the 80s and I'd say, in fact, that it's quite hard to explain what
happened to the UK economy in the early 80s without taking these things into
account. But surprisingly, it was never discussed, even with the
derivations of IS/LM (Mundell-Fleming and IS/LM/BP) that are supposed to deal
with the open economy. Yet it impacts on the validity of the results of
those models.
How does this relate to monetary offset? Well the point is that we can no longer
assume a fixed AS curve when considering the impact of changes in the balance
of fiscal and monetary policy. If
monetary policy is tightened in response to expansionary fiscal policy, we
would expect the exchange rate to appreciate and for the short run AS curve to
move outwards. So even if the result is
to leave the AD curve where it is, there is no way for monetary offset to leave
both inflation and growth unchanged.
I should say that this is as much about flaws in the whole
IS/LM framework as it is about monetary offset.
Trying to extend the framework to incorporate this feedback gets very
messy as it turns out that the position of the whole AS curve depends on where
it crosses the AD curve. It's one of the
reasons I gave up on IS/LM.
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