From the abstract:
How should the Constitution think about “outsourced law enforcement” — that is, investigative activity carried out by private actors that substitutes, in practice, for the labor of law enforcement? Existing doctrine offers a simple answer to this question, centered on chronology. If the government was responsible for outsourcing law enforcement — if a private actor was operating as an “agent or instrument” of the state — Fourth Amendment scrutiny applies, just as it would apply to the conduct of state officials. If, on the other hand, the outsourcing transpired voluntarily — if a private actor decided, without prodding, to assist the authorities — no Fourth Amendment scrutiny applies. This rule is often called the “private search” rule. I adopt that label here.
My goal, in this brief Essay, is to suggest that the private search rule suffers a crucial blind spot — indeed, one that goes to the heart of Fourth Amendment privacy. When it comes to private searches, what we should care about is not which party, private actor or state official, initiated the relationship. What we should care about is whether the private actor, in monitoring others, engaged in privacy-eroding conduct that is functionally similar to — and merits the same regulation as — the privacy-eroding conduct of law enforcement officials. In other words, the relevant question is: Did the labor of a private actor supplant the need for law enforcement involvement at a particular stage of the investigative process? Or, put even more simply: Did a private actor step into the shoes of law enforcement? If so, then Fourth Amendment scrutiny — at least in some measure — is warranted.