If you’re one of the nearly 225 million Americans who owns a smartphone, you know that it is a window into your entire life — your friends, your financial information, your pastimes and, most relevant to the Supreme Court this year, your comings and goings. On Wednesday, the court hears oral arguments in one of the most important privacy cases of the decade: United States vs. Carpenter. The justices are being asked to decide whether the 4th Amendment’s guarantee against unreasonable searches prevents the government from accessing — without a warrant — the data phone companies generate when your phone connects to their cell towers.

Cell tower data are created when you call, text or access a data plan. This means your phone is in near constant contact with towers. Each tower covers a discrete area and, because cell providers now possess so many towers, whenever you move out of one tower’s tracking area, you move into another. The resulting data can form a connect-the-dots picture of where you were when, and it can reveal a lot if someone cares to examine it closely.

Imagine that you are recovering from an opioid addiction that you have hidden from your friends, family and employer. Twice a week you drive to a support group. The police, who are investigating your doctor for over-prescribing, know that you are his patient. They ask your cellphone provider for months of your records, which establish your visits with your doctor and also your regular (and admirable) rehab efforts.

Authorities can often ask your cellphone provider to release the list of every cell tower your phone contacted without ever needing to go before a judge.


Or the government could seek to learn who has been attending political protests or advocacy-group meetings, checking which phones were near a particular tower at a particular time. As the law now stands, authorities can often ask your cellphone provider to release the list of every cell tower your phone contacted without ever needing to go before a judge to get a warrant, which would otherwise require them to show probable cause that a crime has been committed.

There are those who see absolutely nothing wrong with the government being able to request, or even require, such cooperation. The phone was with you in public places. Anyone could have spotted you entering or leaving the doctor’s office or support group. And the records the government is requesting were maintained by a business for its own use. Hardly private, right?

And yet most Americans expect their cellphone location data will remain private. Along with 17 lawyers and legal scholars, we filed an amicus brief in the Carpenter case describing a mountain of research on the privacy beliefs and attitudes of ordinary citizens. The data show that most Americans don’t even know their location information is being collected. And when they find out it is, they do not expect that it can be freely shared. In other words, people are not “knowingly” turning their location histories over to their cellphone providers and “voluntarily relinquishing” their privacy, as some have argued.

That this location information is contained in business records shouldn’t override people’s privacy expectations. Businesses control quite a lot of information that the police cannot access without a warrant. For example, Google and Yahoo can access the contents of their customers’ emails. But there are rules about when and how they can do so, and the courts that have ruled on the question have held that the government needs a warrant to make them turn over the contents of emails to law enforcement. Unsurprisingly, researchers asking whether people expect their emails to be private also get a strong “yes” from their respondents.


Earlier this fall, Chief Justice John G. Roberts Jr. referred to mathematical calculations in a gerrymandering case as “sociological gobbledygook.” Yet for decades courts have looked to social science analyses when considering segregation, discrimination, jury deliberations, intellectual property and consumer protection, among other topics. These data are particularly relevant in a case such as Carpenter, in which judges are tasked with determining whether a “reasonable person” would expect his or her cellphone’s location to remain private. The justices should not merely substitute their own views for the actual voices of the citizens who overwhelmingly reject unfettered access to their location information.

United States vs. Carpenter concerns a man, Timothy Carpenter, who was not an addict seeking relief in secret or a political protester; he was an armed robber convicted in part on the basis of cell tower data. Surely, the government should be able to get this information to prove such cases, shouldn’t it? Of course it should. Right after it gets a search warrant.

If a warrant isn’t required for the Carpenters of the world, it isn’t required for the rest of us either. And the government will remain free to gather far more information about the behaviors and beliefs of its citizens than it should.


Matthew B. Kugler is an assistant professor of law at Northwestern Pritzker School of Law. Sarah O. Schrup is a clinical associate professor of law at Northwestern University and director of the Appellate Advocacy Center.

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