But today's vast international shipping markets and a hugely interconnected global food supply means that cities no longer need to be so dependent on their own country's agricultural production. It is this broken link, Glaeser argues, that has allowed for the puzzling and concerning rise of urban cities in some of the world's most stubbornly poor countries.

As he explains:

Because trade has alleviated the need for agricultural productivity, cities can develop despite enormously poor hinterlands. This creates both an opportunity—the ability to escape terribly poor rural land—and a challenge. Massive cities can develop at far lower levels of income as we see across the world today.

Or, in more concrete terms, "Today, globalization means that Port-Au-Prince can be fed with imported American rice."

Many of these poor megacities have much weaker institutions and more problematic governing structures, according to Glaeser. As a result, they are far less able to handle the thorny problems they face, and their experiences don’t particularly resemble the more positive ones we are familiar with historically.