A couple of years ago, I made fun of the Supreme Court's decision that it's unconstitutional for the police to point infrared scanners at homes to detect heat (from basement marijuana farms, for instance). Some of my ridicule was directed at Eugene Volokh, who defended the decision (Kyllo v. United States) using a wholly (and admittedly) arbitrary "reasonableness" criterion: infrared detector searches are, to him, "unreasonable", whereas searches of neighborhoods for, say, dirty bombs using a Geiger counter (scientifically not terribly different from an infrared scanner) are "reasonable".
Well, now fellow Conspirator Orin Kerr has provided the first plausible justification that I've heard for the Court's Kyllo ruling. He also sees it as part of a recent trend in the Court's Fourth Amendment jurisprudence. Unfortunately, he also sees this trend as a "potentially troubling development".
The justification, according to Kerr, can be deduced from a recent decision (United States v. Place) in which the Court found police searches for drugs using drug-sniffing dogs to be Constitutional. Kerr believes this decision to be part of a pattern, in which the Court determines whether particular searches are Constitutional under the Fourth Amendment based on what information they extract. Drug-sniffing dogs, for example, only determine the presence or absence of drugs--a matter of pure guilt or innocence, which citizens are therefore presumably not entitled to hide from police officers--and are therefore permissible to use in warrantless searches. Infrared scanners, on the other hand, can potentially detect a wide variety of details about the interior of a person's home--including details that the police have no business knowing, and might conceivably abuse upon discovery.
To Kerr, this new trend is a deviation from earlier jurisprudence, which held that the Constitutionality of searches depended strictly on whether they violated some notion of property rights (suitably abstracted to include, for example, rented properties, but not, say, telephone conversations). In his view, the "information-based" criterion is a product of an oft-misinterpreted "privacy doctrine"--that the Fourth Amendment exists to protect individual privacy in general from police snooping--that is in fact, when properly understood, simply a restatement of the narrower, property-rights-based criterion.
Now, whatever else one might say about the "privacy doctrine" approach of considering what information a search extracts, it at least has the virtue of being possibly, conceivably tied to some notion of public opinion on these matters. Judges are not really in a position to know what kind of information the public want the police not to be able to extract without a warrant, but if they were to guess, they might plausibly come up with an answer such as, "information, such as the inner appearance of the suspect's home, as revealed by an infrared scanner, that does not pertain to whether the suspect has committed a crime". On the other hand, a response like, "that which can be obtained without violating the property rights of the individual, defined in a technical, abstract legal sense, and irrespective of its relevance to the suspect's guilt or innocence", is somehow less plausible as a reflection of the public's preferred definition of the bounds of their own protection from police searches.
Of course, if judges--or law professors like Prof. Kerr--wanted their judgments to defer to the popular will, they could always leave it to the people's representatives to decide. And we all know what horrors that would entail....