WASHINGTON - Researchers secretly tracked the locations of 100,000 people outside the United States through their cell-phone use and concluded that most people rarely stray more than a few miles from home.

The surprising results revealed how little people move around in their daily lives. Nearly three-quarters of those studied mainly stayed within a 20-mile-wide circle for half a year.

The report, published today in the journal Nature, opens up the field of human-tracking for science.

"For the first time we have a chance to really objectively follow certain aspects of human behavior," said coauthor Albert-Laszlo Barabasi, director of Northeastern University's Center for Complex Network Research.

The scientists identified the research location only as an industrialized nation. They would not name the private cell-phone company on whose records the study was based.

The researchers used cell-phone towers to track individuals' locations whenever they made or received phone calls and text messages over six months. For 206 cell phones that had tracking devices, they recorded locations every two hours for a week.

Cesar Hidalgo, a coauthor of the study and a physics researcher at Northeastern, said the team did not know the individual phone numbers because they were disguised.

Nor were researchers able to say precisely where people were, just which nearby cell-phone tower was relaying the calls. They started with six million phone numbers and chose the 100,000 at random to provide "an extra layer" of anonymity for the research subjects, Barabasi said.

The study nevertheless calls attention to what experts say is an emerging issue of privacy.

Tracking locations without the phone owners' consent would be illegal in the United States, said Rob Kenny, a spokesman for the Federal Communications Commission. (Consensual tracking is legal and even marketed as a special feature by some U.S. cell-phone providers.)

The study should set off "ethical alarm bells about privacy and trustworthiness," said University of Pennsylvania bioethicist Arthur Caplan.

"My cell phone is personal," he wrote in an e-mail. "Tracking it and thus its owner is an active intrusion into personal privacy."