Glaeser was a decorated economist, but he was no organizer. He spent most of his time teaching and lecturing, or holed up in his comically messy office, thinking big thoughts, surrounded by disjointed stacks of books and stained teacups with dried-out bags still inside them. When Glaeser thought about how problems got solved, he imagined people in imperial columned buildings like the one he worked in on the edge of Harvard Yard.

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And so it fell to people such as Trauss to create a constituency for unbuilt housing. She had a gift for getting attention, showing up to planning meetings in leggings and cowboy boots and delivering lectures on supply and demand at the public-comment microphone. As her profile rose, thanks to media attention and constant tweeting, Trauss discovered she was far from alone. Around the country, Millennials were organizing yes-in-my-backyard (YIMBY) groups and providing a youthful counterbalance to the usual collection of anti-development crusaders who complained about shadows and neighborhood character at city meetings across America.

In 2016, some 200 of these people, representing cities including San Francisco, Seattle, Austin, Minneapolis, and Boston, gathered in Boulder, Colorado for the first national YIMBYtown conference. The conference was held in a Hyatt and had sessions with titles such as “Building a Progressive Urbanist Coalition” and “Reforming the Sacredness of Single-Family Zoning.” It was a voice that was new and needed, but its members were also overwhelmingly well educated and white, limiting its appeal and power.

During a breakout session called “Why Is the YIMBY Movement So White?,” a small subset of people had a tortured discussion about how they could forge alliances with tenant organizations who didn’t like them very much and fought new housing on the grounds that it would foment gentrification and displace low-income people. There would be more YIMBYtowns in the years ahead, and each time this topic would rise to the top of attendees’ concerns.

The third YIMBYtown was held in Boston in 2018, and Ed Glaeser kicked it off with an hour-long presentation of housing data that he traced with a laser pointer. The real action came later.

The very next day, an older tenants-rights organization called City Life/Vida Urbana was hosting the Boston People’s Plan Assembly, to collect neighborhood-level proposals for how to protect tenants from economic displacement. It was held in a church just a few blocks away from the YIMBYtown gathering. The class and cultural differences were clear: Whereas the YIMBY event drew a young crowd, was conducted in English, and gave way to heavy nighttime drinking, this event was older, multilingual, and had a side room with free childcare.

At the end of the assembly, Lisa Owens, the executive director of City Life/Vida Urbana, went to the altar to explain that the group’s supporters were going to march to the YIMBY conference because they had something to say. After a round of cheers, the 200 or so people at the assembly left their pews to form a wobbly line behind a marching band, then proceeded out of the churchyard, down a hill, and disjointedly around a street corner toward Roxbury Community College, where the YIMBY event was wrapping up.