In a fast-paced lecture at the Harvard Graduate School of Design Thursday evening (March 6), Edward Glaeser, the Fred and Eleanor Glimp Professor of Economics, explained what he called the “central paradox” of cities in the postindustrial age.Weaving 21st century Boston, New York, and Detroit into a context of Western cities over the past millennium, Glaeser explained why cities based on ideas have reinvented themselves and thrived, even as cities based on the production of things have faded.He explained why the rust belt still has as many people as it does. He offered a way to look at the question of rebuilding New Orleans. He set forth a number of ideas that would help cities go green, including a carbon tax.He parted company with one of the great 20th century thinkers about how cities work, Jane Jacobs, by calling for larger-scale housing developments that would make it easier for the middle classes to live in the cities.And he illustrated it all with numerous graphics from his research from Harvard’s Rappaport Institute for Greater Boston — charts on which data points swarm like excited bees around the regression lines.The paradox of cities is that even as transportation costs, and especially communication costs, have “vanished,” Glaeser said, cities, at least the ones built on ideas, “are more vital than ever.”As these costs have tumbled, the urban advantage to manufacturing has fallen away. Auto production has moved from Detroit to its suburbs, and from there to right-to-work states and Asia, Glaeser noted.“The death of distance that killed Detroit,” Glaeser said, “has helped cities that specialize in connecting smart people.” And connecting smart people turns out to be a very essential urban function — one unlikely to disappear as a result of new communications technologies.“How much of what you know comes from face-to-face communication?” he asked. The answer, implicitly, was a great deal. Earnings in the cities are high, because that’s where people go to learn new skills that raise their value in the marketplace. And it’s not just the learning in formal situations like universities; it’s in the informal learning on the job that is important. “Cities are forges of human capital.”This is the key to success for contemporary cities, he suggested. Thus Milan has thrived while Turin, still bound to manufacturing, has faded; Birmingham has reinvented itself and Manchester hasn’t. Minneapolis has prospered, and Cleveland hasn’t really.For some of the fading communities, the question isn’t why they’re losing people but rather why they still have as many as they do. The explanation has to do with housing prices. Housing is a durable good, and a good many people are “captive” of (often quite well-built) homes priced below their replacement value.Glaeser spoke of the development of Chicago in two modes — city of things and city of ideas. Chicago developed as a food center. Iowa corn fed the pigs that came to the Chicago stockyards — “Pigs are corn with feet,” Glaeser explained, to chuckles from his audience in Piper Auditorium. In the days of relatively expensive transportation and no refrigeration, hogs on the hoof were a good way to get the food value of the Iowa cornfields to the consumers in the big cities.But while Chicago was busy serving as hog butcher to the world, it was also the site of what Glaeser calls a “cluster of genius” in the field of architecture. He spoke of the development of the skyscraper as an example of how these clusters function.“The things that happened in Chicago in architecture in the late 19th century are as miraculous a set of intellectual connections as you can possibly imagine,” he said, invoking the names of Jenney, Burnham, Sullivan, and Wright.“It’s frustrating to have this lengthy debate about who built the first skyscraper. … It’s clear that there were several different people experimenting with the same thing.“That is the point about urban clusters of genius. There isn’t one person. As in almost every creative field, ideas are jointly produced. …“It’s a collective enterprise. I feel quite positive that that is the right way to understand the creation of the skyscraper in 19th century Chicago.”Turning to the “green cities” of the 21st century, Glaeser introduced another paradox: the “high environmental cost of environmentalism.” By this he referred to the way some of the most progressive communities are making it hard to build. This makes it hard for the middle classes to afford homes in urban areas, and tends to further development in car-dependent metropolises — he cited the big four of Atlanta, Dallas, Houston, and Phoenix. “This is where Jane Jacobs was wrong,” he said, citing her opposition to large-scale development, which he said was needed to welcome the middle classes into the cities.Glaeser warned against a “false dichotomy” between “people” and “place,” and he clearly doesn’t mind if some places lose population. Speaking of the reconstruction of New Orleans after Hurricane Katrina (a subject on which, he allowed, he has “gotten into trouble” before), he commented on the “huge amounts” of money being devoted to that enterprise and suggested that the right question to ask is: “What would be the best thing for the people who used to live there?”Upcoming lecture: ‘Integrating Sustainability in Design, Planning, and Project Funding,’ 6:30 p.m. March 19, Piper Auditorium, Gund Hall, Harvard Graduate School of Design.