1. 4th Amendment (Pre-Internet)
Robert Sloan
Richard Warner

2. 4th Amendment
“The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.”

3. The Warrant Requirement
A warrant is a written order signed by a court authorizing a law-enforcement officer to conduct a search, seizure, or arrest.
Probable cause exists when the facts and circumstances within the police officer's knowledge provide a reasonably trustworthy basis for a man of reasonable caution to believe that a criminal offense has been committed or is about to take place
See Carroll v. United States, 267 U.S. 132 [1925].

4. The Warrant Requirement
Particularly describing: Warrants must provide enough detail so that an officer with the warrant can ascertain with reasonable effort the persons and places identified in the warrant. 
Neutrality; The magistrate before whom an officer applies for a warrant must be neutral and detached.

5. What The 4th Amendment Protects
Hoffa: The 4th Am. “protects the security a man relies upon when he places himself or his property within a constitutionally protected area, be it his home or his office, his hotel room or his automobile. There he is protected from unwarranted governmental intrusion. And when he puts something in his filing cabinet, in his desk drawer, or in his pocket, he has the right to know it will be secure from an unreasonable search or an unreasonable seizure.”
Warren Court: Warren was Chief Justice from 1953 to Brown, Gideon, Miranda, Mapp v. Ohio, Griswold

6. What Is Protected?
Places: The 4th Am. “protects is the security a man relies upon when he places himself or his property within a constitutionally protected area, be it his home or his office, his hotel room or his automobile. There he is protected from unwarranted governmental intrusion.”
Hoffa v. United States

7. Places in Hoffa
“The petitioner was not relying on the security of the hotel room; he was relying upon his misplaced confidence that Partin would not reveal his wrongdoing.”
“The risk of being overheard by an eavesdropper or betrayed by an informer or deceived as to the identity of one with whom one deals is probably inherent in the conditions of human society. It is the kind of risk we necessarily assume whenever we speak.”

8. Reasonable Expectations
Here is one way to make sense of this.
Hoffa (subjectively) expected that Partin would not reveal the information to law enforcement.
This expectation was not objectively reasonable because ““risk of being betrayed by an informer is the kind of risk we necessarily assume whenever we speak.”
Consider an analogy.

9. An Analogy
Ralph trusts Roger, so he confides in him that he is being unfaithful to his wife.
He subjectively expects Roger will not tell his wife.
Roger tells Ralph’s wife. Was Ralph’s expectation objectively reasonable?
That depends on what Ralph expects and on what the standard of “objectively reasonable” is.

10. What Ralph Expects
Suppose Ralph expects that there is no possible future in which Roger tells Ralph’s wife.
That expectation is not objectively reasonable precisely because “risk of being betrayed by an informer is the kind of risk we necessarily assume whenever we speak.”
But of course this is not what Ralph expects. Compare your expectation that you will not be hit by a car when you cross the street.

11. Background Assumptions
You make various background assumptions when you cross the street: about how drivers behave, about your eyesight, and ability to detect dangers. Relative to these assumptions, you assume you will not be hit.
Hoffa no doubt did the same. Relative to assumptions about Partin’s trustworthiness, he assumed he would not be betrayed.
How is that not objectively reasonable?

12. A Normative Question “Objectively reasonable” is a normative question.
Compare: “ You can’t expect me not to tell your wife that you are having an affair. She is my friend too!”
Does not deny that Ralph expects Roger not to tell.
Does not deny that Ralph has good reasons—from Ralph’s perspective.
Denies that Ralph ought not to expect what Roger does.

13. The Normative Question in Hoffa
When Hoffa tells Partin things that are evidence of crimes, ought the state be able to introduce those conversations into evidence in court?
If we answer “Yes,” then on that normative ground we can say that Hoffa ought not to expect Partin not to reveal the conversations.
The next case, Katz, addresses this issue.

14. Katz v. United States
Katz transmitted “wagering information by telephone from Los Angeles to Miami and Boston in violation of a federal statute.”
The police collected evidence by attaching a listening device to the outside of a phone booth.
The police argued that a phone both was not a place protected by the 4th Amendment.
Katz argued it was.

15. The Majority Position
The majority rejects both arguments on the ground that “the Fourth Amendment protects people, not places. What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection. But what he seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected.”
The court holds a warrant was required.

16. Harlan’s Concurrence
The majority did not provide much of a rationale for its decision. Justice Harlan’s concurrence does:
the subjective expectation/objectively reasonable expectation approach.
Note: Neither the majority nor Harlan explain why Katz’s expectation of privacy was objectively reasonable.

17. Possible combinations of subjective and objectively reasonable expectations
of privacy
Expectation
of lack of privacy
No expectation
Objectively
reasonable
Not objectively
reasonable
Objectively
reasonable
Not objectively
reasonable
Objectively
reasonable
Not objectively
reasonable

18. 4th Amendment after Katz
Expectation
of lack of privacy
Expectation
of privacy
No expectation
Objectively
reasonable
Not objectively
reasonable
Objectively
reasonable
Not objectively
reasonable
Objectively
reasonable
Not objectively
reasonable
Nonexistent case?
Red Arrows: Current 4th Amendment analysis track.
The 4th Am. cases never discuss the possibility of no indeterminate expectation, as may be the case with pay-with-data exchanges.  You either expect privacy or not in the cases; they do not divide the "not" into "expect not" and "not expect either way."  Second, in the "expect no privacy" cases, it is irrelevant that it might be reasonable objective reasonable for certain things to be private.  The lack of a subjective expectation just decides the issue.
Irrelevant under Katz?

19. 4th Amendment after Katz
Created by
Disclosure to
3rd parties?
4th Amendment after Katz
Expectation
of lack of privacy
Expectation
of privacy
No expectation
Objectively
reasonable
Not objectively
reasonable
Objectively
reasonable
Not objectively
reasonable
Objectively
reasonable
Not objectively
reasonable
Nonexistent case?
Current doctrine is the disclosure to 3rd parties (such as your bank by virtue of having an account) eliminates any subjective expectation of privacy and waives 4th Am. protections.  Marshall (in his Maryland v. Smith dissent) and Soto Major (in her Jones concurrence) question whether the doctrine is appropriate. 
Created by
Disclosure to
3rd parties?
Irrelevant under Katz?

20. What Is A Search?
4th Am.: “The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures . . .”
Hoffa did not involve a search, but Katz did.
After Katz: A search for 4th amendment purposes is a search for things or data that (1) one expected would remain private, (2) which expectation was reasonable.
Or so it would seem.