IT'S not at all clear that falling commodity prices - or the Reserve Bank's latest cut in the official interest rate - will lead to a lower Aussie dollar. But if it doesn't fall, and the economy doesn't look like it will stay growing at its trend rate, the Reserve will just keep cutting rates.
The best way to think of the Reserve's problem in using monetary policy (interest rates) to maintain non-inflationary growth is that for some years it's been trying to keep the economy on an even keel while we're being hit by two powerful, but opposing economic shocks: the expansionary shock from the resources boom and the contractionary shock from its accompanying very high exchange rate.
This involves predicting, then continuously monitoring the relative strengths of the opposing forces, with the objective of keeping inflation in the 2 to 3 per cent range and the economy growing at about its medium-term trend rate of 3.25 per cent a year - neither much less than that nor much more (because the economy's already close to full employment).
For most of last year the Reserve's greatest worry was that the stimulus from the resources boom, applied to an economy already near full employment, would push up inflation. By November it realised any inflation threat had passed - it was actually falling - whereas growth was on the weak side of trend. It therefore cut the cash rate by 125 basis points (1.25 percentage points) between November and June.
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The latest reassessment of the balance between the two conflicting forces bearing on the economy is that the fall in coal and iron ore prices and the shelving of expansion plans by some second-tier miners will cause mining investment spending to peak in the middle of next year, a little earlier and a little lower than expected.
This pushed the growth forecast for the overall economy a bit below trend. Hence this month's rate cut.
It's reasonable to attribute the Aussie's remarkable strength since the start of the resources boom predominantly to our high commodity export prices and vastly improved terms of trade.
That makes it reasonable to expect the fall in export prices would lead to a commensurate fall in the Aussie, thus reducing its contractionary effect on our export and import-competing industries.
But the historical correlation between our terms of trade and our exchange rate, while strong, can also be quite loose for fairly long periods. So it's not surprising the Aussie has held up so well.
The plain truth is that, because financial markets simply aren't as ''efficient'' as it suits some economists to believe, no theory they can come up with adequately and always explains the Aussie's ups and downs.
The best you can say is the terms-of-trade theory works well a lot of the time and over the medium to long term, while its main rival - that the Aussie's driven by the size of the ''differential'' between our interest rates and those on offer in the big economies - sometimes works at other times.
So it's equally unsurprising the 150-basis-point fall in our official interest rate since November has done little or nothing to get the Aussie down.
My guess is the Aussie could stay much where it is for years to come. Why? Because of a third, omnibus theory: those countries with the best growth prospects tend to have strong exchange rates whereas those with poor prospects tend to have weak exchange rates.
It's a safe bet that, even if we were to fail in our attempt to get growth back up to trend, our prospects will stay a mighty lot better than those for the United States and Europe. Then there's our AAA sovereign credit rating and exposure to the fastest-growing region, Asia, to entice capital inflows.
Remember, exchange rates are relative prices, so if some countries are down, others must be up. We're not alone in the strong-currency boat: there's also Switzerland, Canada, New Zealand and Sweden.
So, what if the Aussie stays up and its contractionary effect on the economy remains undiminished? The Reserve would just keep cutting the official rate until it foresaw growth getting back up to trend.
In principle, the only thing that could deter it from this response is rising inflation pressure, but with growth below trend that's hardly likely.
Its goal wouldn't be to get the dollar down (though that would be welcome) so much as to stimulate the presently ailing, interest-sensitive parts of the domestic economy: home building and commercial (as opposed to mining-related) construction.
This would quite possibly get house prices growing reasonably strongly which, in turn, could perk up consumer confidence, particularly for people in or near retirement.
But that's actually the vulnerability in such an approach by the Reserve. Returning to a period of exceptionally low mortgage interest rates risks igniting another credit-fuelled boom in property prices, at a time when many households remain heavily indebted and most foreign economists can't understand why we haven't had a property bust.
The rich world spent most of the 1970s, '80s and '90s worrying about how to get inflation down to acceptable levels. We now know that, particularly since the advent of independent central banks and inflation targeting, the central bankers have got (goods-and-services price) inflation licked.
One small problem: as the global financial crisis so powerfully reminded us, in the process they've aggravated the problem of asset-price inflation, with its huge bubbles and terrible busts.
To date, the world's monetary economists are at a loss on how they can control goods-price inflation and asset-price inflation at the same time.
