What does it mean for someone to be displaced by gentrification? And in a just world, what do our cities’ neighborhoods look like?

As reported by Next City, a team of researchers at the University of California-Berkeley has put together a an analysis that probes just those questions. But the stilted answers they come up with get at the heart of one of the greatest contradictions in the debate over where America’s cities are going, and where they ought to be.

Let’s back up for just a moment. The Urban Displacement Project, led by Miriam Zuk, wants to use predictive modeling to peer into the future of neighborhood change in the Bay Area. Drawing on previous theory, Zuk and her colleagues categorized every Census tract in the region into one of eight stages of gentrification, and used previous trends to project how much further gentrification might go.

So far, so good. But digging into the Urban Displacement Project’s methodology reveals a curious choice.

In their course of their analysis, Zuk and her team try to quantify the number of people who have been or might be “displaced” as a result of gentrification. How do you know if someone has been displaced? As the authors point out, previous researchers have generally cast a wide net: displacement might occur as a result of any number of factors, from disinvestment that makes a home unpleasant or dangerous to rising prices that force tenants to look for more affordable housing elsewhere.

But rather than attempt to sort through different types of change, the Urban Displacement Project simply assumes that any reduction in the number of low-income people in a neighborhood is a result of displacement. As Zuk acknowledges, this is rather extreme: it doesn’t leave room for someone to move away simply because, as people do, they decide that they would be happier living somewhere else; and it would define a low-income person who remained exactly in the same place but got a better-paying job as a “displaced person.” It’s unlikely that, in the teeth of the recession, this last number is particularly high—but unless we believe that economic progress in poor neighborhoods is impossible, it’s hard to see how this could be a viable assumption going forward.

This notion of displacement also seems to imply that neighborhoods are, or ought to be, entirely static and that their residents never move. We know that this isn’t true for neighborhoods in general, and poor neighborhoods in particular. As the Urban Institute has shown, one of the main ways that poor families improve their economic situation, get access to better schools and reduce their risks of crime is to move to different neighborhoods. As the Berkeley team writes, it’s probably not fair to describe those sorts of moves as totally “voluntary,” since the movers are reacting to deficits beyond their control in communities they may otherwise want to remain in. But in a context that’s heavily focused on gentrification, rolling this other kind of “displacement” may obscure more than it illuminates.

Neighborhood demographic change, after all, mostly depends on two things: who’s moving in, and who’s moving out. That means a neighborhood’s poverty rate will remain the same only if the poverty rate for each of those two groups is exactly the same. It also means that this balance can be thrown off by a change in either group. If the people moving in become less poor, the neighborhood’s poverty rate will fall even if nothing has changed about the people moving out. Even if, that is, people are not being displaced at any greater speed than they were before.

Much of the research on gentrification has focused on trying to untangle these two threads. In doing so, it has often found that neighborhood change is driven as much or more by a change in the “move-in” group than the “move-out” group. One of the most famous studies, by Columbia’s Lance Freeman, found that low-income residents of gentrifying neighborhoods were only somewhat more likely to move than low-income residents of non-gentrifying neighborhoods: the real difference was who was coming in to replace them. The Urban Displacement Project counts this shift in in-migration as displacement—which doesn’t leave room to interpret, say, a decline in neighborhood stigma that causes middle-income people to stop shunning a poor neighborhood as a positive development.

But the most fundamental problem with this definition of displacement is what it implies about the kinds of neighborhoods we want in our cities. If every reduction in the number of poor people in a neighborhood is “displacement”—and we agree that displacement is something to be avoided—then the only conclusion is that every neighborhood must remain exactly as poor as it is now.

Claiming that every neighborhood ought to stay as it is with respect to poverty—or, for that matter, racial segregation, although Zuk et al are mostly focused on economics—would be one thing in a country where the status quo was relatively egalitarian, and where the neighborhoods that people lived in reflected their own agency in a just, fair, and free society. But obviously wherever that country is, we do not live in it. When the status quo is the result of generations of discrimination and cruelty, taking Zuk’s position is chaining ourselves to a continuing legacy of terrible injustice.

In fact, a full 88 of the 129 tracts that the Urban Displacement Project identify as “gentrifying” are still poorer than the Bay Area as a whole. Obviously that doesn’t mean they’ll stay that way; gentrification and displacement is a process along a spectrum. But it does demonstrate how much room there is in many American neighborhoods to reduce the poverty rate without becoming an exclusionary bastion of privilege. The question is whether, once the process has begun, it can be arrested before housing prices push out all or nearly all of the original community of residents. The answer to that is not obvious everywhere, though our report, Lost in Place, showed that nationally, instances in which this process of improvement managed to reduce the poverty rate in previously high poverty neighborhoods are extremely rare. Despite high-profile examples of neighborhood change in some coastal cities, it’s not at all clear that there’s necessarily a gentrification “tipping point” that leads inexorably to the exclusion of all the poor.

None of this is to minimize the fact that, especially in places like San Francisco, rising home prices often constitute an injustice. Clearly, being forced to leave your home is wrong, even if you live in a neighborhood that has been a target of economic or racial segregation. But we need a framework for understanding housing and neighborhood change that allows us to talk about and address the full range of challenges our cities face. A definition of “displacement” that implicitly rejects any change to the status quo is not up to that task.

Instead, we need to ask what will lead to more open, diverse and integrated neighborhoods in our metropolitan areas. Just as it’s reasonable and necessary to ask how we’ll promote greater housing choices in the nation’s wealthy suburbs (a discussion provoked by the Obama administration’s Affirmatively Furthering Fair Housing rule), it’s necessary to think about what rectifying the pattern of segregation in America’s poor neighborhoods would look like as well.