What makes a city thrive?

The population density of Paris is about three times that of Boston. Does this mean Paris is three times as much fun as Boston, or that if Boston’s population were compounded by three, it would become another Paris?

These questions are not as silly as they may sound – that is, if you start from the premise, shared by many architects, urban designers, planners, and others, that density is the secret to creating healthier urban environments – with safer streets, better schools, more vibrant cultural and commercial activity, more efficient public transportation, nicer parks, and a less degraded environment.

On Jan. 28, the Kennedy School’s Rappaport Institute for Greater Boston in cooperation with the Boston Society of Architects (BSA) held a forum at Harvard to examine the concept of density and to look at ways it could be achieved. The forum was a follow-up to an earlier conference held by the BSA in September 2003.

While most of those who attended the event viewed density as a cure for the city’s ills, that attitude is by no means shared by all who take an interest in the welfare of America’s towns and cities. The forum’s title reflected that ambivalence: “The ‘D’ Word: What it Takes to Build Dense Communities in Greater Boston.”

In certain ways, the debate between density and its opposite – sprawl – has been going on for the past half-century, said Charles Euchner, the executive director of the Rappaport Institute, who moderated and helped organize the event.

“There’s been an ongoing struggle to strengthen the city core, but not always to increase urban density. What’s new, I think, is that we’re realizing cities are really about people. The more people you bring in, the more vibrant the city will become,” Euchner said.

While it might not quite stack up to Paris, it can hardly be said that Boston is lacking in vibrancy. And yet, if present trends continue, there may be trouble ahead. Over the past 40 years, Boston’s population has gone from 800,000 to 600,000. The drop would have been even sharper if it were not for an increase in immigration since 1965.

According to the pro-density argument, urban institutions require a certain threshold population to support them. If not enough people want to shop or eat out, there won’t be many good stores or restaurants. If the audience for music, theater, or art is small, these activities will not flourish. If the tax base is scanty, schools and municipal services will be substandard. Even parks need people to use them, and if the parks are deserted, they will not receive the upkeep they need to remain attractive.

Density is also considered good for the environment because it is easier and cheaper to provide heating, electricity, sewerage, and other services to people living in concentrated groups than to those in single-family homes in suburban areas. As a result, the impact of dense populations on the surrounding environment is less harmful.

If density is such a good thing, why haven’t more people gotten the message? The first panel took up this question. David Parrish, who described himself as a “recovering architect,” now a senior vice president at the Federal Home Loan Bank of Boston, brought up the point that whether people want to take advantage of the social and cultural advantages of the city or seek the sprawling suburbs depends on where they are in the life cycle.

“A wise old man once told me, if you’re looking for someone to have sex with, you live in the city; if you’ve got someone to have sex with, you live in the suburbs. Sometimes I think it may be as simple as that.”

Others spoke about the stigma associated with density.

“Density used to be associated with poverty,” said BSA President David Dixon. “We are part of that baby boom generation whose parents moved away from poverty by moving away from the city. The core reason for the stigma is a historical artifact.”

Hubert Murray, a Boston-based architect and urban planner, spoke about the visual images that the word “density” brings to many people’s minds.

“It’s the turn-of-the-century photographs that Jacob Riis took of the slums on the Lower East Side, it’s the Pruitt-Igoe housing project in St. Louis that had to be torn down in the 1970s. It’s the image of the huddled masses, of disease, crime, congestion. But the statistics show that density is not a leading factor for these conditions. For example, the TB statistics show that the key elements are new immigrants and poverty. Similarly, it’s poverty and the lack of job opportunities that drive crime and not density, per se.”

Alfred Wojciechowski, a principal in a Boston design firm, said that for density to work, people have to feel comfortable moving out into the public realm.

“It’s very much about walkability. Goods and services have to be immediately available. You have to be able to walk to them.”

Wojciechowski quoted design theorist Christopher Alexander, who called the ineffable combination of factors that make a neighborhood pleasant the “quality without a name,” sometimes abbreviated by designers as “QWAN.”

Developer Roger Cassin agreed. “A five-minute walk in a neighborhood where you feel uncomfortable is too far.”

In many cases, achieving greater density means building taller buildings, something that often proves unpopular, especially in the Boston area.

“We’re an awfully conservative city when it comes to high buildings,” said David Lee, a partner in a Boston architectural firm and an adjunct professor at the Harvard Graduate School of Design. “Anything over three stories and people come out and rebel against it. We’re still stuck in the 19th century. We haven’t even made it into the 20th.”

“The dominant religion has always been the religion of short buildings,” said Dixon. “I challenge anyone to claim that their lives have been harmed by a tall building.”

The second panel concentrated on how public policy could be used to achieve greater density. Stephanie Pollock of the Conservation Law Foundation said that the debate over density needed to be reframed.

“Public policy is the art of solving problems, but density is not a problem that anyone wants to solve. In order to inject density into the public policy debate, we must propose it as a solution to other problems.”

Edward Moscovich, an independent economist and consultant, identified one such problem.

“In the commonwealth as a whole, most public officials know that development should take place someplace else. If residential development is proposed, their job is to stop it. Why? Because it costs about $6,000 per year to educate a child, and they know the real estate taxes on a $300,000 home won’t cover it.”

The result of this reluctance to build housing results in housing shortages, which results in higher housing costs, which tends to reduce density. Some members of the panel spoke about using incentives to counter this tendency.

Barry Bluestone, an economist who directs the Center for Urban and Regional Policy at Northeastern University, suggested that the state government offer cities a $2,000-$3,000 density bonus for each unit of housing built to help cover the cost of schools and other services.

What chance is there that Boston and other communities will begin to see density as the solution to the problems they face? Euchner believes that conferences like this one may help to move decision makers in the right direction.

“Communities are constantly debating the level of development they should allow. I think forums like this one create greater literacy and awareness about this subject, and when people can talk intelligently, it can have an enormous impact on the debate. I think people are more open to density than they were a couple of years ago.”