Are you one of those people who think of big cities as little more than hotbeds of pollution, crime and social inequalities? Well, think again. A new report in this week's Proceedings of the National Academy of Sciences USA confirms what many city dwellers, who account for the bulk of people on Earth, have claimed for years: Cities have an almost magical ability, spurred by increased human interaction, to stimulate innovation and increase wealth.

The report also pooh-poohs the popular comparison of the growth of cities with biological organisms. An animal slows as it balloons in size ; in contrast, the researchers note, cities speed up as population and everything from crime to per capita income grow.

Cities create a sort of "urban economic miracle," says study co-author Luis Bettencourt, a research scientist in Los Alamos National Laboratory's Theoretical Division. "When you integrate all these people and all these activities and the struggle to make a living, total productivity increases," he says.

Bettencourt and his colleagues at Arizona State University (A.S.U.), Dresden University of Technology in Germany and New Mexico's Santa Fe Institute, modeled the growth of a city according to three categories of factors: material infrastructure (road surfaces, electrical cable, etc.), human needs (such as total energy consumption and housing) and patterns of social activity, including total bank deposits, research and development, new cases of AIDS and new patents filed. The researchers sifted through an extensive amount of data on many urban systems—mostly big American cities, but also European (primarily German) and Chinese urban areas.

The researchers mathematically modeled these factors according to population growth to see how each respond when more people move to a city. They found that human needs, such as employment, utility consumption and housing, correspond directly with the population: As the number of people doubles so does the need for housing, jobs and electricity infrastructure, which encompasses the number of roads, gasoline stations and the like already in place and does not necessarily keep pace with individual growth—the ratio of user to facility simply rises. (And so, for example, there are simply more customers at the available gas stations.) At the other extreme, researchers found that increases in social activity and production outpace population growth. In other words, if the number of city denizens doubles, these factors—both negative (crime) and positive (wealth creation, total wages and gross domestic product)—will more than double.

"These scaling laws give you some suggestion of …[how] … your city will behave as it grows," in terms of economic activity, resource consumption, etc., Bettencourt says, adding that smaller cities, like Portland, Ore., and huge epicenters, like New York City, fall along the same continuum and are subject to the same multipliers.

"The practical application of this work is that the problem is not large cities, the problem is the conditions in which some people live in large cities," says study co-author Jose Lobo, an economist at A.S.U.'s School of Sustainability in Tempe. "Policies should be directed to making large cities more livable"—for instance, enacting legislation or spending money to alleviate poverty and crime, the negative effects of growth.

Thomas Parris, director of sustainability programs at iSciences, a Burlington, VT, research company dedicated to improving understanding of sustainability, agrees that the main message of the paper is a recharacterization of cities so that better decisions can be made as urban areas continue to grow. "This is a fascinating paper that quantitatively explores the complex interactions between urbanization, sustainability and social innovation," he says. "Insights, such as those presented in this paper, will help guide our collective choices as the pace of socioecological change accelerates."