Do We Need a New Fourth Amendment?

Slobogin's book offers a new conceptualization of the Fourth Amendment rooted in what he calls the proportionality principle: An investigative technique should be permitted under the Constitution only if the strength of the government's justification for the technique is roughly proportionate to the level of intrusion it causes . Slobogin roots this principle in Terry v. Ohio and its pragmatic balancing of law-enforcement and privacy interests. To determine how much justification the Fourth Amendment requires, Slobogin argues, courts should assess the intrusiveness of the investigatory technique and then set a proportionate threshold of proof that the government must show. The more intrusive the technique, the higher must be the degree of ex ante certainty established before the technique can be used.

Moderately intrusive steps might be permitted with a court order merely establishing relevance, while more intrusive steps might be allowed only with probable cause. This case-by-case balancing of interests should replace the bifurcated design of existing Fourth Amendment law that leaves some practices entirely unregulated by the Fourth Amendment and then requires warrants based on probable cause for others.