Terrorist watch lists are probably a good idea. But because the government is, well, the government, it doesn't really care whether the watch list is right. Their incentive is to make sure that absolutely nobody who might be a terrorist is able to do absolutely anything in the US.

These lists always have to trade off between Type I error (false positive) and Type II error (false negative). Since we live in an imperfect world, you have to decide whether to err on the side of ensnaring the innocent, or missing the guilty.

But people building terror watch lists only get punished if they introduce Type II error. Hence, they have no incentive to avoid Type I error. Which is why they have an easy procedure to put people on, and no obvious way to remove them again.

The rest of us, however, have to trade off the risk of getting killed in a terrorist attack, and the risk of never being able to fly or have a bank account again. With something like one in three hundred Americans on watch lists, it seems to me that we have gone too far.

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