There is an unfortunate logic to bank runs. The rational thing for any depositor to do is to join in the run if there is even the smallest possibility that the bank really is in danger of failing. If you pull your money out and the bank survives, the costs of your mistake are tiny compared to the costs if the bank fails with your money in it.


Right now in Europe, there are trots — think of them as slow-speed runs — not on individual banks but on national banking systems. Depositors are pulling money out of banks in countries that may or may not be solid, and putting the money into German banks.


That move sets in motion a daisy chain of loans that leaves Germany’s central bank, the Bundesbank, financing the system.


An interesting question is whether the rate of withdrawals is accelerating.


We don’t know. These are numbers that come out with long and variable delays of weeks to months.


Certainly the numbers were large before worries about Spanish banks intensified in May.


It is conceivable that the trot could speed up even more before the Greeks get around to their next election, two weeks from Sunday.


Before considering whether the numbers are scary — and there is a sharp debate on that among economists — consider how the daisy chain works. An Italian withdraws 1,000 euros from his local bank and deposits them in a German bank. A lot of other Italians — and Irish and Greeks and Spaniards and Portuguese — do the same. Of course, those local banks still have the same loans outstanding.


If enough deposits leave, the banks may have to borrow from their own central banks, such as the Bank of Italy.


If other Italian banks do not have excess reserves — and in this situation they will not — the Bank of Italy then borrows from the European Central Bank. And where does the European Central Bank get the money? As it happens, the German banks now have more deposits than they need, and they deposit money with the Bundesbank. The Bundesbank lends the money to the European Central Bank.


Those loans between central banks are called Target 2 flows, a term that comes from the name of a payment system. When things were running smoothly, no one paid much attention to them. Now many worry about them.


All told, it appears that close to a trillion euros in Target 2 loans were owed to the European Central Bank by central banks of countries with leaky banking systems at the end of April. I say appears because two debtor central banks — those of France and Austria — have yet to report April numbers. A year ago, the figure was under 500 billion euros.


The largest lender to the European Central Bank under the program — 644 billion euros at last count — is the Bundesbank. But the national banks of Luxembourg, Finland and the Netherlands are also substantial creditors.


Broadly speaking, those are the countries that have prospered as others lost competitiveness. If the euro zone is to survive, they are also the ones most likely to have to finance a solution. Let us pause for a second and ask what the advantages and disadvantages are for those who move their deposits. They will get no interest from a German bank, but they got none from their local banks. They will give up having a convenient local branch. They have no currency risk, since their deposits are still in euros and they can write euro checks to pay their bills.


But what will happen if the euro blows up? Talk of that has intensified in recent weeks, as it became clear that the German diet of constant austerity was not attractive to voters in other countries. If the euro zone did blow up, or shrink, would depositors in Spain see their euro deposits converted to pesetas at some unknown exchange rate?


If you think the answer to that question may be yes, what do you have to lose by moving your money? That is the logic of a bank run.


Legally, no country can leave the euro zone. The euro was supposed to be the Roach Motel of currencies: once a country checked in, it could never check out. But now it is hard to see how some countries’ economies can ever become competitive if they cannot devalue their currencies, and voters are growing restless.