The Supreme Court established a test for determining when a government search triggering Fourth Amendment protections has occurred in Katz v. United States (1967). The majority ruled that a government intrusion into an area where a person has a reasonable expectation of privacy constitutes a search within the meaning of the Fourth Amendment and thus requires a warrant issued upon a showing of probable cause.

Justice Harlan's concurrence in Katz, though not binding as it was not issued as the majority opinion, has provided the generally accepted two-prong test for determining when a reasonable expectation of privacy exists. Harlan defined an REP as when an individual has (1) displayed a subjective expectation of privacy (2) that society is prepared to recognize as reasonable. (1) and (2) are regularly referred to as the subjective and objective prongs respectively.

For an REP to exist, an individual must meet both prongs. To meet the subjective prong, the individual must make an effort to withdraw their activities from public view. If I close the curtains on my front windows, I have displayed a subjective intent to conceal whatever occurs behind those windows from public view. But that subjective intent must also be reasonable in the eyes of society.

For example, I can't walk into a crowded room filled with cops and scream, "No one listen to me!", proceed to discuss a plan to commit murder, and then claim I had an REP. I displayed a subjective intention to achieve privacy, but it is not one which society would view as objectively reasonable.

Following this line of analysis, the Supreme Court held in Smith v. Maryland (1979) that any information committed to the care of a third-party is not protected by the Fourth Amendment. In Smith, the Court evaluated whether an individual had an REP in his pen registers (phone company records of the phone numbers dialed from a particular phone line). The Court held that there is no REP where a person voluntarily exposes their information to public view because that person has not displayed a subjective intent to assert their privacy.

The doctrine may have been on shaky ground even back in 1979, but certainly the justices deciding the case could not possibly have imagined the world we live in now, where we routinely store our most private information on servers in the hands of third-parties.

Clearly the doctrine needs some rethinking. As it stands, law enforcement can access incredibly detailed personal information about a person without a warrant. It is nearly impossible to show a subjective expectation of privacy while still functioning as a member of society since we are so dependent on phones.

Personally, I would start by arguing that company privacy policies and password protections indicate a subjective intent to remove electronically-stored personal information from general view. The shear quantity of data being stored indicates to me that any reasonable person would never expect another individual to actually inspect their emails or cell phone location data. We are voluntarily committing the information to unthinking computers which are theoretically secure against unauthorized human intrusion, not to actual human beings.

But my instinct is to go farther. The amount of information about an individual collected by networked devices and corporations increases exponentially every year. With the internet-of-things and wearable tech on the rise, the problem is only going to grow. Cell phone location data, heart rate measurements from fitness trackers, dietary information from calorie-tracking apps, complete and detailed purchasing histories from credit card statements, statuses and pictures posted on social media, etc. etc. etc. Each individual datum may be meaningless on its own, but like a jigsaw, when put together they can reveal the most intimate details of a person's life. And there's essentially no way to opt out.

So I think we need an opt out method. I think people should be able to assert that the information resulting from a private commercial arrangement is private between the parties and that this should have equivalent effect to a confidentiality privilege (like doctor-patient or attorney-client privilege). A company's terms of service could then include a clause that says something like, "Any information gathered during the provision of these services is to remain private between the parties." If the government can get a warrant, then they can access the information. With so many multiplying avenues for individuals to unintentionally give up their privacy rights, there should be a simple, straightforward method for individuals to reassert those rights.