It occurs to me that in our taxonomy of varieties of public goods, we are missing a category.  Normally we distinguish public goods according to whether they are rival/non-rival and whether they are excludable/non-excludable.  It is generally easier to efficiently finance excludable public goods because people by the threat of exclusion you can get users to reveal how much they are willing to pay for access to the public good.

I read this article about Piracy of sports broadcasts and I started to wonder what effect it will have on the business of sports.  Free availability of otherwise exclusive broadcasts mean that professional sports change from an excludable to a non-excludable public good.  This happened to software and music but unique aspects of those goods enable alternative revenue sources (support in the case of software, live performance in the case of music.)

For sports the main alternative is advertising.  And the only way to ensure that the ads can’t be stripped out of the hijacked broadcast, we are going to see more and more ads directly projected onto the players and the field.

And then I started wondering what would be the analogue of advertising to support other non-excludable public goods.  The key property is that you cannot consume the good without being exposed to the ad.  What about clean air?  National defense?

But then I realized that there is something different about these public goods.  Not only are they not excludable– a user cannot be prevented from using it, but they are not avoidable — the user himself cannot escape the public good.  And there is no reason to finance unavoidable public goods by any means other than taxation.

Here’s the point.  If the public good is avoidable, you can increase the user tax (by bundling ads) and trust that those who don’t value the public good very much will stop using it.  Given the level of the tax it would be inefficient for them to use it.  Knowing that this inefficiency can be avoided you have more flexibility to raise the tax, effectively price discriminating high-value users.

If the public good is unavoidable, everyone pays whether you use ads or just taxation (uncoupled with usage), so there really isn’t any difference.

So this category of an avoidable public good seems a useful one.  Can you think of other examples of non-excludable but avoidable public goods?  Sunsets: avoidable.  National defense:  unavoidable.

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