Spy in your pocket: cellphone tracking.

• Author: Sullum, Jacob
• Position: Citings

In July the New Jersey Supreme Court ruled that police generally need a warrant to obtain information about the locations of cellphone users. Less than two weeks later, the U.S. Court of Appeals for the 5th Circuit said just the opposite.

The first decision was based on Article I, Paragraph 7 of the New Jersey Constitution, while the second decision was based on the Fourth Amendment to the U.S. Constitution. But those provisions are virtually identical, banning "unreasonable searches and seizures" of "persons, houses, papers, and effects." The crucial difference between the two decisions is the "third party doctrine," which holds that people have no constitutional right to privacy with respect to information they voluntarily share with others.

Based on a series of Supreme Court decisions beginning in the early 1970s, federal courts are bound by this principle, which New Jersey courts have emphatically rejected. Hence the New Jersey Supreme Court had no difficulty concluding that the government may not demand cellphone location data at will. "Disclosure of cell-phone location information, which cell-phone users must provide to receive service, can reveal a great deal of personal information about an individual," the court noted. "Yet people do not buy cell phones to serve as tracking devices or reasonably expect them to be used by the government in that way. We therefore find that individuals have a reasonable expectation of privacy in the location of their cell phones under...