by Cass Sunstein

In the aftermath of the disclosure of the terrorist plot in London, many people have been calling for a complete ban on carry-on luggage. For those who want such a ban, it is one way to ensure that Americans are "safe." The editorial writers at The New York Times proclaim, "the surest way to keep dangerous materials out of the cabin is to keep virtually all materials out of the cabin." But there is a big problem here. We cannot know whether to ban carry-on luggage without exploring (a) the degree of risk reduction that we would get and (b) the costs and burdens that we would impose. Suppose that the risks of carry-on luggage are essentially zero (in light of other security precautions) and that the costs of a ban would be very high--not only in terms of inconvenience, but in sheer monetary terms as well (because of lost productivity, including reductions in air travel). Consider the fact that the surest way to prevent terrorist attacks on airplanes is to ban air travel--but the costs of such a ban would greatly outweigh the benefits in terms of risk reduction. The analysis is unquestionably closer for a ban on carry-on luggage, but it is necessary to undertake that analysis--not to support precautionary steps simply because they are precautionary steps. There is a more general point here. For risks associated with both terrorism and environmental harm, some people argue on behalf of the most aggressive precautions on the ground that they are more aggressive and therefore more protective than current alternatives. But it is always necessary to ask: From such precautions, how much do we get, and how much do we lose? Often precautions actually make us less safe, not much so. (Those who drive, to avoid the risks associated with flying, usually increase their own risks.) My uninformed hunch is that a ban on carry-on luggage cannot be justified and that it may even increase overall risks. But the broader point is that it is quite hopeless to support the ban on the grounds that it is "the surest way to keep dangerous materials out of the cabin."