According to Hampton, our tendency to interact with others in public has, if anything, improved since the ‘70s. The P.P.S. films showed that in 1979 about 32 percent of those visited the steps of the Met were alone; in 2010, only 24 percent were alone in the same spot. When I mentioned these results to Sherry Turkle, she said that Hampton could be right about these specific public spaces, but that technology may still have corrosive effects in the home: what it does to families at the dinner table, or in the den. Rich Ling, a mobile-phone researcher in Denmark, also noted the limitations of Hampton’s sample. “He was capturing the middle of the business day,” said Ling, who generally admires Hampton’s work. For businesspeople, “there might be a quick check, do I have an email or a text message, then get on with life.” Fourteen-year-olds might be an entirely different story.

Philadelphia was the only location of the four where Hampton found more people by themselves than the P.P.S. films did. But Hampton claims the effect was offset by something more profound: The apparent uptick could be explained in part by an increase in the number of women on the street, many of them alone.

In fact, this was Hampton’s most surprising finding: Today there are just a lot more women in public, proportional to men. It’s not just on Chestnut Street in Philadelphia. On the steps of the Met, the proportion of women increased by 33 percent, and in Bryant Park by 18 percent. The only place women decreased proportionally was in Boston’s Downtown Crossing — a major shopping area. “The decline of women within this setting could be interpreted as a shift in gender roles,” Hampton writes. Men seem to be “taking on an activity that was traditionally regarded as feminine.”

Across the board, Hampton found that the story of public spaces in the last 30 years has not been aloneness, or digital distraction, but gender equity. “I mean, who would’ve thought that, in America, 30 years ago, women were not in public the same way they are now?” Hampton said. “We don’t thinkabout that.”

It was so gradual a change that it was barely noticeable as it was happening.