Look around, and YIMBYs are a growing presence. There’s a YIMBY group in Somerville, Massachusetts, and one in Los Angeles; there’s a San Francisco YIMBY party and a YIMBY group in Portland. YIMBYtown, a national conference, will take place in Oakland this month; Helsinki is hosting Yimbycon in August.

“We’re saying yes to opportunity, in contrast to the gloom-and-doom negativity of NIMBY,” says Jesse Kanson-Benanav, who is the chair of A Better Cambridge, an advocacy group that works to get more housing built in Cambridge, Massachusetts, and who will be attending YIMBYtown along with a handful of other Boston organizers. A Better Cambridge, which was founded in 2012 to advocate for more housing, pushed back against concerns from the community about noise, height, and traffic. Kanson-Benanav’s group was instrumental in getting a 19-story development approved in Cambridge’s Central Square this year. “There’s now a pro-housing, pro-density constituency in Cambridge that hadn’t been at the table previously,” he told me.

Similar groups have seen successes in other cities. In Colorado, Boulder Forward, a YIMBY group that organized the inaugural YIMBYtown conference last year, organized opposition to a ballot measure that would have restricted growth in town. Residents voted down the ballot initiatives by a 2-to-1 margin. And in Berkeley, East Bay Forward, a YIMBY group, protested at a city-council meeting at which downzoning—a change to zoning policy that would restrict the amount of housing that could be built—was on the agenda. The city council decided not to consider the measure at that meeting, which the YIMBYs considered a victory.

These housing advocates have fine-tuned their message as they’ve gained momentum. Many of them do not make a point of emphasizing that building housing increases affordability. What has been more successful, YIMBY groups are recognizing, is appealing to people’s sense of equity and fairness. Economic mobility is stunted throughout much of the country, and one reason for this is that low-income people can’t afford to live in high-opportunity cities such as San Francisco and New York. Taking pressure off of housing prices by building more units is one way to open up cities’ economies to more people.

“I think there are more people understanding housing as a social-justice issue,” Gabriel Metcalf, the president and CEO of SPUR, the San Francisco Bay Area Planning and Urban Research Association, told me. “While they might not like their communities changing with higher-density buildings, more people understand that they are necessary to live up to our values as progressives.”

There’s no doubt that affordability is an issue in some of the country’s biggest metropolitan areas. But talking about affordability isn’t necessarily what gets people to support building more housing, and research backs up this approach that many YIMBYs take. In a recent working paper, Clayton Nall, a professor of political science at Stanford, and William Marble, a graduate student there, found that most people are ambivalent when they’re told that developments will reduce the cost of housing. In some cases, if they’re homeowners, such information will make them less likely to support development, likely because they think the values of their own properties will go down.