Why is Spain paying higher interest rates on its government debt than the UK? The answer to this question is illuminating: membership of a currency union makes a country fiscally fragile. This is inherent in the construction: members are neither sovereign states nor components of a federation. The big challenge for the eurozone is to resolve this contradiction.

In an important paper, Paul de Grauwe of Leuven university notes this contrast between the current positions of Spain and the UK. The yield on Spanish government 10-year bonds is almost two percentage points higher than that on UK equivalents, at 5.3 per cent against 3.5 per cent. This is a bigger difference than it may seem. If one assumes that the Bank of England and the European Central Bank both meet their 2 per cent inflation target, Spain’s real interest rate is more than double that of the UK.

Do the fiscal positions of the two countries explain the contrast? Not obviously: Spain will have lower ratios of net and gross public debt to gross domestic product until at least 2016. It will also have lower fiscal deficits until 2014 and a lower primary fiscal deficit (before interest payments) until 2013. (See charts) True, according to the International Monetary Fund, the UK fiscal deficit is forecast to be 1.3 per cent of GDP in 2016, against Spain’s 4.6 per cent. And differences in primary deficits explain 2.9 percentage points of this gap. But even this is not solely due to a difference in fiscal effort, since Spain’s economy is forecast to grow on average by 1.6 per cent between 2011 and 2016, while UK average growth is forecast at 2.4 per cent.