SANTA FE, New Mexico, April 5 (TNSres) -- Santa Fe Institute issued the following news release: Humans are becoming more urban, with more than half of the world's population now living in cities. This rapid growth poses unique challenges to both the study and governance of cities -- a challenge made harder because we lack a single common definition of "city." Cities are often defined by political or administrative units or by built-up areas identified via satellite imagery. The best definitions  more