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Whr R U?

4 June 2008 (All day)
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Evlin M. During; (inset) Cesar A. Hidalgo and Marta C. Gonzalez/Northeastern University

On the move.
By following the paths of individual cell phone users (inset), scientists have found that humans may obey well-defined rules of motion.

Some of us are homebodies, and some are jet setters. But a new study suggests that people, regardless of travel habits, follow the same general patterns of motion, spending the bulk of their time in a few favorite spots. The conclusions, obtained by tracking thousands of mobile phone users, could help researchers devise more accurate models of disease outbreaks.

Human travel routines affect how a virus will spread during an epidemic. But monitoring large numbers of people isn't easy. A 2006 study using hundreds of thousands of dollar bills as surrogates found that a typical bill circulated within a small area but could also travel long distances when its owner went on a trip or vacation (ScienceNOW, 25 January 2006). It wasn't clear whether this pattern reflected the movements of individuals, however, because a dollar bill can easily change hands.

So physicist Albert-László Barabási and colleagues at Northeastern University in Boston turned to the most ubiquitous of human accessories: the cell phone. They monitored 100,000 users for 6 months and recorded the location of the cell phone tower that transmitted each call or text message. The team found that most people stayed close to home, and a select few regularly took long trips. This mixed profile suggests that the typical journey seen in the dollar bill study had at least partially captured the movements of multiple people rather than individuals, the team reports today in Nature.

The researchers then ranked each user's favorite spots and found that regardless of how mobile they were, people returned over and over to a few top locations with similar probability. For example, two users would have roughly the same chance of being found in their third-favorite spots, whether it was the gym or the theater. These hangouts were often located near the path between their top two destinations--usually home and work. "It indicates that there's something fundamental in the way we move around," says Barabási.

Daniel ben-Avraham, a physicist at Clarkson University in Potsdam, New York, thinks the decision to track cell phones is "brilliant. If you really want to study mobility of people, that's the way to do it," he says. But Stephen Eubank, a physicist at Virginia Polytechnic Institute and State University in Blacksburg, says scientists can't assume that the patterns found in the study will hold true under different conditions. Changes such as the collapse of a bridge or the rising price of gasoline, he says, could alter people's behavior in ways that would affect their overall movements.

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