An example of societal mobility: 

Recent Census data show that in a given year, roughly 16% of the U.S. population moves and that about half of those moving leave the counties they had been living in.  Consequently, about 8% of the U.S. population changes its source of drinking water every year.  Some of those 8% are "repeat movers", but there is obviously a significant number of people who do not drink from the same source of water for 70 years (the time period typically considered when establishing safe drinking water standards).