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“As smoking gives us something to do with our hands when we aren’t using them, Time gives us something to do with our minds when we aren’t thinking,” Dwight Macdonald wrote in 1957. With smartphones, the issue never arises. Hands and mind are continuously occupied texting, e-mailing, liking, tweeting, watching YouTube videos, and playing Candy Crush.

Americans spend an average of five and a half hours a day with digital media, more than half of that time on mobile devices, according to the research firm eMarketer. Among some groups, the numbers range much higher. In one recent survey, female students at Baylor University reported using their cell phones an average of ten hours a day. Three quarters of eighteen-to-twenty-four-year-olds say that they reach for their phones immediately upon waking up in the morning. Once out of bed, we check our phones 221 times a day—an average of every 4.3 minutes—according to a UK study. This number actually may be too low, since people tend to underestimate their own mobile usage. In a 2015 Gallup survey, 61 percent of people said they checked their phones less frequently than others they knew.

Our transformation into device people has happened with unprecedented suddenness. The first touchscreen-operated iPhones went on sale in June 2007, followed by the first Android-powered phones the following year. Smartphones went from 10 percent to 40 percent market penetration faster than any other consumer technology in history. In the United States, adoption hit 50 percent only three years ago. Yet today, not carrying a smartphone indicates eccentricity, social marginalization, or old age.

What does it mean to shift overnight from a society in which people walk down the street looking around to one in which people walk down the street looking at machines? We wouldn’t be always clutching smartphones if we didn’t believe they made us safer, more productive, less bored, and were useful in all of the ways that a computer in your pocket can be useful. At the same time, smartphone owners describe feeling “frustrated” and “distracted.” In a 2015 Pew survey, 70 percent of respondents said their phones made them feel freer, while 30 percent said they felt like a leash. Nearly half of eighteen-to-twenty-nine-year-olds said they used their phones to “avoid others around you.”

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It is the troubling aspects of social…