In San Diego, half the neighborhoods are inhabited by mostly poor residents. During my recent stint as director of planning and economic development there, residents would often ask me how I planned to revitalize those neighborhoods. I tended to give an alarmingly honest answer: I wasn’t sure.

Officials who work in urban planning use every tool at their disposal to improve neighborhoods and cities. We strive to make them more healthful, equitable and attractive places to call home. But no matter how many transportation or development investments we put into some struggling neighborhoods, we can’t do the one thing that would most improve residents’ lives: put money in their pockets.

That represents something of a conundrum for those of us in the urban business. Today we’re celebrating the revival of cities and city neighborhoods. The population of big cities in the United States is going up for the first time in my lifetime. Urban neighborhoods across the country in cities large and small are teeming with life in a way that we haven’t seen for decades. Yet for every urban neighborhood that’s reviving, there are probably two or three that are still struggling or even declining. There’s considerable evidence, for example, that the concentration of poverty is growing rapidly. And even as mayors across the country report a general economic revival, they also say that the revival is uneven.

In other words, cities are no different from the nation as a whole. The rich are getting richer while everybody else is muddling along. But in a city, this inequality plays itself out in very stark geographical terms: Some neighborhoods stay rich, some neighborhoods turn around, some neighborhoods stay poor and some poor neighborhoods get poorer.

While mayors from Seattle’s Ed Murray to New York’s Bill de Blasio have highlighted this inequality, is there really anything they can do about it? Is the rich-versus-poor division in our cities today the result of an overall trend toward inequality, or is it somehow part of the cause?

In large part, the answer boils down to the old people-versus-place question in urban policy: Do you focus on improving struggling neighborhoods in the hopes that everyone in the neighborhood will be better off? Or do you focus on helping people get a leg up, even if it means they leave the neighborhood? There’s considerable evidence that poor people living among more affluent folks generally do better. But there’s probably no way to move all people of modest means to affluent suburbs -- and it’s really hard to encourage affluent folks to move into a poor neighborhood without starting a cycle of gentrification.

Ultimately the solution likely lies in creating more economic opportunity -- not just improvements to the physical environment -- in poor neighborhoods. The ongoing frustration is whether that’s even possible when society at large is becoming so unequal and there are so few paths to upward mobility in any location. So while there’s no doubt that place-based efforts are integral to improving any neighborhood, urban planners can’t forget broader economic issues, such as job creation, that influence the shape of cities. Unless people who live in their neighborhoods have a way up, all those community development efforts may not do much good.