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How do you know if someone’s your friend? Ask your cell. Because your phone knows who your friends are. Sometimes even before you do. Or so says a report in the Proceedings¹ of the National Academy of Sciences.

Scientists who study social networks have long been hampered² by one thing: their subjects are not always reliable reporters. They don’t lie about their associations, but their ability to recall how much time they spent with Tom, Dick or Cody last month is not always accurate.

So scientists have been searching for a better way to track relationships. Which is where mobile phones come in. Researchers handed nearly a hundred subjects souped-up cell phones that recorded information about calls, text messages and even how physically³ close callers were to those they contacted. Analyzing⁴ calling patterns, the investigators⁵ were able to infer which contacts were friends with 95 percent accuracy. In some cases, the patterns revealed a friendship in the making months before people declared someone a pal⁶.

The data could also predict job satisfaction: people who spend all day on the phone with friends, it seems, are generally not stoked about their work. So remember—keep your friends close. And your cell phone even closer.