Commentary Migration seems to be a constant in history. Migratory trends fluctuate, but they inevitably surface somewhere as human beings seek greener pastures and a better life. A new migratory trend is emerging within our country: More people are moving out of large American cities and fewer are moving in; hence, some urban populations are starting to shrink. The potential ramifications of this trend are ominous. Before I write about what’s at stake for cities and the people who reside in them, let’s look at the lifecycles of cities and the reasons why an increasing number of people are either exiting cities or deciding not to move into them. The primary factors that impel the growth and development of cities are of an economic nature. I don’t mean to discount or devalue social considerations; human beings are social creatures and we crave contact with others, but without a viable economic …