Megan McArdle on just what externalities are and why we pay attention to them:
For those who might not know the term, “externality” is economist-speak, and it means about what it sounds like: an effect that your action has on others. An externality can be positive or negative, and obviously, we as a society would like to have as many as possible of the former and as few as possible of the latter. In other words, “Your right to swing your fist stops at the end of my nose.”
I’m a libertarian, and libertarians love talking about externalities. They give us a (relatively) clear way to define what are and are not legitimate scopes of public action. Whatever you’re doing in the privacy of your own bedroom with another consenting adult is really none of my business, even if I think you oughtn’t to be doing it. On the other hand, if you’re breeding rats and cockroaches in there, and they’re coming through the shared wall of our respective row houses, then I have the right to get the law involved.
Framing things as “externalities” is therefore a good way to get a libertarian, or someone who leans that way, on your side. And such frames have come up over and over in the debate over Obamacare, which has been variously justified by the cost to the state of emergency room care; the cost to society of free-riding young folks who don’t buy insurance until they get sick; the public health cost of people who don’t go to the doctor and get really, expensively sick; an unhealthy workforce that is less productive; and the cost to friends and relatives who have to chip in to cover uninsured medical expenses.
I didn’t find any of those arguments particularly convincing. The third can just be dispensed with on the grounds of accuracy: In general, preventive medicine does not save money. Oh, it may save money in the particular case of someone whose diabetes or cancer went long undiagnosed. The problem is, you can’t just look at the cost of sick folks who would have been a lot cheaper to treat if their conditions had been caught earlier. You also have to include the cost of all the healthy people you had to screen in order to catch that one case of disease. And with limited exceptions, the cost of screening the healthy generally outweighs the cost of treating the chronically ill. Now, you can certainly argue for preventive care on other grounds — for example, that it makes people healthier (though even then you have to add the cost of unnecessary medical procedures, such as biopsies following a false positive on a blood test, which is why we do not, say, give annual mammograms to every American woman). But it’s not generally a money saver, so this particular externality doesn’t exist.
The rest of the arguments have some weight, but in the end, I don’t think they’re weighty enough. Let me explain.