Some commenters on my Europe/euro post offer a reductio ad absurdum: if Spain should have its own currency, why not every state/town/family in America?

Strange to say, economists have thought about that — a lot. It’s called optimal currency area theory. (Optimal? Optimum? Nobody seems to know — or care).

The basic idea is that there’s a tradeoff. Having your own currency makes it easier to make necessary adjustments in prices and wages, an argument that goes back to none other than Milton Friedman. As opposed to this, having multiple currencies raises the costs of doing business across national borders.

What determines which side of this tradeoff you should take? Clearly, countries that do a lot of trade with each other have more incentive to adopt a common currency: the euro makes more sense than a currency union between, say, Malaysia and Ecuador. Beyond that, the literature suggests several other things that might matter. High labor mobility makes it easier to adjust to asymmetric shocks; so does fiscal integration.

When EMU began as a project, there were a number of studies comparing the EU with the United States. What all of them suggested was that Europe was less suitable as a currency area, basically because of lower labor mobility and lack of fiscal union. That didn’t settle the question of whether the euro was a good idea, but it did suggest that appealing to the success of the United States with a single currency didn’t tell you much.

What I’ve always found interesting is the way many Europeans now insist that a single currency is absolutely essential, when the example of Canada — which is closer to the United States than it is to itself — provides an obvious counterexample. But people tend to forget that Canada exists …