Theory vs. Practice

However, as ever with exchange rate models, in an open economy with high capital mobility there remains the issue of delay in the transmission mechanism. Monetary models suggest that an increase in interest rates should lead to an increase in the investor’s weighting of interest-bearing securities and a corresponding reduction in the weighting of money/cash. This in turn should lead to a reduction in the demand for and therefore the price of goods, which according to PPP should result in an offsetting appreciation of the nominal exchange rate in order to restore equilibrium.
In practice, it may not take place exactly like this, at least in the short term. Say you are an investor in US Treasuries and the Federal Reserve tightens monetary policy by increasing interest rates. Depending on what were market expectations for Fed policy prior to that and also depending on where you were positioned on the US yield curve, you may be facing losses on your position due to the simple inverse relationship between bond yields and bond prices. Eventually, the incentive to hold interest-bearing securities will rise as interest rates rise, but only at the point where the investor believes interest rates have stopped rising. Until that time, the investor may in practice do the opposite of what the model suggests, by reducing their position in interest-bearing securities and reverting to money/cash in order to preserve capital. Theoretically, the investor will have more money/cash to spend on goods and this should push up prices, which in turn should lead to depreciation — rather than appreciation — of the exchange rate according to PPP to restore equilibrium.
Equally, the natural reaction of our US Treasury investor to a fall in interest rates is not necessarily to reduce the position, given that falling yields equal rising prices. Eventually, the reduction in income will not be offset by the capital gain, at which point the investor will indeed reduce the position in favour of other assets such as money/cash. Before that, they may well maintain or even increase the position in interest-bearing securities in order to reap the capital gains impact. Thus, a reduction of interest rates may at least initially lead to an actual reduction in money/cash within portfolios, in turn causing money demand and prices to fall and the currency to appreciate according to PPP to restore equilibrium.
I suspect that the very suggestion that a reduction in interest rates may lead to a reduction rather than an increase in money/cash may cause one or two economists reading this to foam at the mouth. The point is a serious one however, and it is this — the assumption that a change in monetary policy leads directly and automatically to a parallel change in the exchange rate is flawed for the following reasons:
There may be a delay in the transmission mechanism
The initial exchange rate reaction may be the exact opposite of what standard models assume This is not in any way to reduce the importance of the original work. Rather, it is to bring it into the context of modern-day trading and investing conditions. Over the medium to long term, the Mundell–Fleming model of policy combinations is an invaluable guide to future exchange rate direction. In the short term, however, as I have tried to show, there may be delays and distortions, which at least put off the anticipated results.