Living in bigger, denser settlements allowed the inhabitants of ancient cities to be more productive, just as is true for modern urbanites, according to a new study by scientists at the University of Colorado Boulder and the Santa Fe Institute. As modern cities grow, they obey certain rules. As the population increases, for example, the settled area becomes denser instead of sprawling outward. This allows people to live closer together, use infrastructure more intensively, interact more frequently, and as a result, produce more per person.