Geotagging reveals not only where you are, but also people you might know

If you see Fred and Susie standing in the same line at the cafeteria just once, it probably doesn't mean anything. If they show up together in many different places, it starts to mean a lot. But how many times do you have to see them together before it becomes significant? Surprisingly few, say Cornell computer scientists. And that could mean your online presence tells the world more than you intended.

Comparing the locations of photos posted on the Internet with social network contacts, researchers found that as few as three "co-locations" at different times and places could predict with high probability that two people posting photos were socially connected. The results have implications for online privacy, the researchers say, but also suggest a quantitative answer to a very old psychological question: What can we conclude from observing coincidences?

"This a kind of question that goes way back," said Jon Kleinberg, the Tisch University Professor of Computer Science, who conducted the study with Dan Huttenlocher, the John P. and Rilla Neafsey Professor of Computing, Information Science and Business and dean of the Faculty of Computing and Information Science. "Online data gives us new ways to address it."

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