Discussions about the Fourth Amendment sometimes include arguments that look like this:
The Fourth Amendment protects reasonable expectations of privacy. I expect privacy in _________, and I’m pretty reasonable, so I have a reasonable expectation of privacy in ________ and the Fourth Amendment should therefore protect it.
This reasoning is really common, but it’s also based on a misunderstanding. It is based on the erroneous belief that the test for whether government conduct violates a “legitimate” or “reasonable” expectation of privacy under the Fourth Amendment is whether a reasonable person would expect privacy in what the government learned. I can certainly understand why a lot of people think that way. After all, the test is “reasonable expectation of privacy,” which sure sounds like it should be based on whether a reasonable person would expect privacy. But it turns out that this isn’t how the Fourth Amendment works, and I wanted to explain why.
The key idea is that the phrase “reasonable expectation of privacy” is a constitutional term of art, much like other constitutional terms of art like “due process,” “free speech,” “strict scrutiny,” or “fundamental rights.” You normally can’t answer if something violates “due process” just by asking if there was a “process” that seemed “due,” much like you can’t answer if a regulation satisfies “strict scrutiny” by scrutinizing the regulation while being “strict.” In all of these cases, the phrase captures a complicated body of caselaw; You can’t just repeat the name for the test and think that the name accurately captures the doctrine. Rather, you need to recognize the test as a term of art that reflects a large body of caselaw, and you need to get the answer from the caselaw.
In the case of the “reasonable expectation of privacy” test, the Supreme Court has repeatedly refused to provide a single test for what makes an expectation of privacy “reasonable.” Instead, it has used different approaches in different settings. In some settings, an expectation of privacy becomes reasonable when it is a good thing as a policy matter for the Fourth Amendment to protect it. In other settings, an expectation of privacy becomes reasonable when it is backed by positive law outside the Fourth Amendment. In other settings, an expectation of privacy becomes reasonable when it shields the government from particularly private facts. Finally, in some settings, an expectation of privacy is reasonable when common social norms make exposure jarring or unlikely. I have called these four approaches the Four Models of Fourth Amendment Protection; they are four different ways of interpreting what makes an expectation of privacy reasonable, and they each apply in various degrees in different factual settings.
It sounds complicated, I realize, and it is. But I think there are good practical reasons why the Supreme Court has refused to provide a single test for what makes an expectation of privacy reasonable; no one test consistently and accurately distinguishes government investigative steps that need Fourth Amendment regulation from those that don’t. (See the Four Models paper linked to above starting at page 25 for the detailed answer of why.) Whatever the reason, the key point is that a “reasonable expectation of privacy” is not just an empirical question into whether a reasonable person would expect privacy. You can’t just mull over the words “reasonable expectation of privacy” and derive an answer from those words any more than you can mull over words like “due process” and derive an answer from for what the due process clause forbids.