More on the US bank tax

Further to my last post, Greg Mankiw — who is not a man to lightly advocate an increase in taxes on anything, but who understands very well the problems of negative externalities and implicit guarantees — has written a good post on the matter:

One thing we have learned over the past couple years is that Washington is not going to let large financial institutions fail. The bailouts of the past will surely lead people to expect bailouts in the future. Bailouts are a specific type of subsidy–a contingent subsidy, but a subsidy nonetheless.

In the presence of a government subsidy, firms tend to over-expand beyond the point of economic efficiency. In particular, the expectation of a bailout when things go wrong will lead large financial institutions to grow too much and take on too much risk.
What to do? We could promise never to bail out financial institutions again. Yet nobody would ever believe us. And when the next financial crisis hits, our past promises would not deter us from doing what seemed expedient at the time.

Alternatively, we can offset the effects of the subsidy with a tax. If well written, the new tax law would counteract the effects of the implicit subsidies from expected future bailouts.

My desire for a convex (i.e. increasing marginal rate of) tax derives from the fact that the larger financial institutions are on the receiving end of larger implicit guarantees, even after taking their size into account.

Update:  Megan McArdle writes, entirely sensibly (emphasis mine):

That implicit guarantee is very valuable, and the taxpayer should get something in return. But more important is making sure that the federal government is prepared for the possibility that we may have to make good on those guarantees. If we’re going to levy a special tax on TBTF banks, let it be a stiff one, and let it fund a really sizeable insurance pool that can be tapped in emergencies. Like the FDIC, the existance of such a pool would make runs less likely in the shadow banking system, but it would also protect taxpayers. Otherwise, with our mounting entitlement liabilities, we run the risk of offering guarantees we can’t really make good on.

I agree with the idea, but — unlike Megan — I would allow some of it to be collected directly as a tax now on the basis that the initial drawing-down of the pool came before any of the levies were collected (frustration at the political diversion of TARP funds to pay for the Detroit bailout aside).

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