Cities seem to form when state population surpasses 10,000. Over 5000 years, top city populations have remained 0.1−0.5% of world population. The square root law of city population: The largest city population tends to be hundred times the square root of state population. This law held from 3000 BCE to 1800 CE. Constantinople exemplifies it from 400 to 1800. It no longer holds, as the economic reach of major cities surpasses international borders.