He says cities are also better for the environment.

EDWARD GLAESER: "There is significantly less carbon usage in cities. There are two reasons for that, one of which is less driving; they are more likely to use public transportation. And when they drive, they drive shorter distances. And the second is that people in the cities occupy smaller homes than people living in rural areas."

Not surprisingly, Professor Glaeser thinks even more people should move to cities. He says developing high-rise buildings, or "building up," is a way to avoid developing wider areas, or "building out."

STEVE EMBER: But architect and urban designer Michael Mehaffy says research suggests that the buildings do not have to be very tall.

MICHAEL MEHAFFY: "It might only require four, six, eight stories, something like that, to get very good urban densities and to have a very vital urban environment."

Mr. Mehaffy also says high-density living does not always improve quality of life.

MICHAEL MEHAFFY: "There is a point where more density doesn't really get you very much. I mean, it can be very helpful in some circumstances, it can be very destructive in some circumstances. I think we should really focus on what urban living gives to us in the network of relationships, not so much an abstract number like density and 'Let's just make it absolutely as high as possible and let's have tall buildings.' Because once you do that, you start to kick in a lot of negative effects from density."