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People move like predators

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7:57pm, March 18, 2008
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From New Orleans, at a meeting of the American Physical Society

Every morning is the same: You get up, shower, and eat breakfast, and you're ready to go. On a hunt.

If hunting is not in your daily routine, it might as well be, or so it could seem to someone tracking your movements. A new study based on data from cell phone use shows that people's daily roaming mirrors familiar patterns—universal statistical laws that researchers have observed before in the movements of certain carnivores looking for prey.

Albert-László Barabási and Marta González of Northeastern University in Boston and their collaborators sifted through 6 months' worth of text messages and call records for 100,000 users, provided by cell phone companies. By tracking which cell phone towers users were connecting to at any given time, the data allowed researchers to map individuals' movements throughout the day, as long as they went far enough to enter another tower's service area.

González says that a better understanding of human mobility might help improve urban planning and model how infectious diseases spread.

Some of the conclusions were hardly shocking. For example, most people commute, some farther than others, and a few people travel a lot, while others don't travel at all.

But, seeking statistical patterns, the researchers measured the distances between consecutive calls and calculated which distances were traversed with higher probability. Predictably, shorter trips happened more frequently than longer ones, but anomalously long "jumps" took place relatively often, following a statistical law called a Lévy distribution.

Researchers have observed Lévy motion in other animals including, most recently, in several species of marine predators, as described in the Feb. 28 Nature. Occasional longer trips into new areas, the thinking goes, increase the chances of finding food.

While some people in the study were much more mobile than others, the less mobile ones had similar behavior, just on a smaller scale. "Each trajectory is statistically identical," which is surprising, Barabási says. "I would have expected that people who travel a lot would have significantly different patterns."

Dirk Brockmann of Northwestern University in Evanston, Ill., says the new study is unprecedented in the richness of its data. Humans, he says, must be following some strategy that leads to a Lévy distribution, but what the strategy is remains a mystery. "Why is there a mathematical law?" he says. "There still is no plausible explanation."

On the other hand, H. Eugene Stanley of Boston University says the similarity between humans and other animals is not surprising. After all, he says, the same instinct that pushes a predator to try new territory in search for food could also inspire people to venture into a new neighborhood hoping to get a new job or to discover a good bar. It's a jungle out there.

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