And, perhaps most importantly for the immediate future, it's almost exactly the list of cities where college grads have been moving since the recession began.

The Increase in Population of College Grads Between 2007 and 2012, by City

This is a tight feedback loop. The densest cities tend to be the most educated cities, which are also the richest cities, and often the biggest cities. They’re gobbling up a disproportionate share of college grads. And, as a result, they are becoming richer, denser, and more educated.

This feedback loop goes by many impressively multisyllabic names—geographic sorting, economic agglomeration, cumulative advantage. But they're all fancy ways to describe something simple. Even as older and less educated Americans are moving to the suburbs, young people with college degrees are moving toward density, and their migratory patterns are encouraging future young people to follow in their steps.

Is the sheer size of these cities’ educated workforce providing a kind of automatic marketing to new college grads? Or are denser cities just more productive, so they have more jobs to dangle in front of twentysomethings? Yes and yes, perhaps. Density and degrees have a magical relationship. Economists Edward Glaeser and Matthew Resseger have shown that in cities with lots of college grads, more-populous cities have more-productive workers. But the magic ends when you take away the degrees: Metro areas with fewer college grads show no relationship between population and productivity.

This week for me is bookended by two trips to New Orleans and Miami to look at and discuss economic opportunities and those cities. Both have good news. Despite its population decline, New Orleans has actually seen an increase in college grads moving to the city, greatly raising its share of bachelor’s degrees. According to 2012 Census figures, nearly a quarter of its adult population is under 34, more than New York or San Jose. Miami has the second-most-diverse Millennial cohort in the country and the number of young people living within three miles of its downtown has doubled in the last 10 years.

But many cities that don't already appear at the top of these degrees-and-density lists are fighting migratory currents that are pulling more of the most talented young people to the same small set of cities. “At the same time that American communities are desegregating, racially, they are becoming more segregated in terms of schooling and earnings," Enrico Moretti wrote in his book The Geography of Jobs. In other words, today's richest cities might not mimic the collapses suffered by the richest U.S. cities from the 1970s. "The knowledge economy has an inherent tendency toward geographical agglomeration," he wrote. "Initial advantages matter, and the future depends heavily on the past.”