Standard theories of monetary union suggest that they work best when the participating countries experience the same shocks. They work least well when they experience asymmetric shocks. I have always found it difficult to teach this, because until now the EU's monetary union has not really had to deal with a big shock. The fall out from the US sub-prime crisis is imposing an asymmetric shock on euroland. Consequently, we now begin to see the dilemma that EMU creates for its members and its single central bank.
Germany, in contrast, is struggling with rising inflation: "The hard-line bloc [is] led by the two German council members, Bundesbank chief Axel Weber and the ECB's chief economist Jurgen Stark. The latest spike in oil and food costs has pushed German inflation to 3pc, the highest since the launch of the euro and fast approaching the level where it may erode popular support for the currency."
Thus, one monetary policy but divergent economic developments across euroland. Someone has to accept a monetary policy that not only fails to address their current needs but will actually further worsen their situation. The dilemma is complicated by uncertainty; the more German unions question whether the ECB will use policy to keep inflation down in Germany (i.e., the more they believe that monetary policy will target Spain and the Med) the larger the nominal wage increases they will seek. Hence, to keep inflation down in Germany, the ECB must be hard line and build a reputation. But, being willing to raise interest rates to build this reputation risks making things even worse for "club med."
Not surprisingly, this "technical decision" is spilling over into politics, as French and Italian politicians have chastened Trichet for the hard line he is adopting.
It is precisely this problem that caused me to write, more than ten years ago, that EMU is not obviously a very good idea.