  A quantitative understanding of cities' demographic dynamics is becoming a
potentially useful tool for planning sustainable growth. The concomitant theory
should reveal details of the cities' past and also of its interaction with
nearby urban conglomerates for providing a reasonably complete picture. Using
the exhaustive database of the Census Bureau in a time window of 170 years, we
exhibit here empirical evidence for time and space correlations in the
demographic dynamics of US counties, with a characteristic memory-time of 25
years and typical distances of interaction of 200 km. These correlations are
much larger than those observed in an European country (Spain), giving to the
US a more coherent evolution. We also measure the resilience of US cities to
historical events, finding a demographical post-traumatic amnesia after wars
(as the Civil War) or economic crisis (as the 1929 Stock Market Crash).
