Up, then down
Growing knowledge and wealth increase the options available to individuals. Personal needs and wants outweigh communal concerns, but eventually even communal goods become affordable. Despite increasing population and personal consumption, U.S. environmental pollution has mostly improved since 1970, when per capita income exceeded $15,000 in 1998 currency. A consensus of opinion set the direction, technology and wealth provided the means.
Source: U.S. Statistical Abstract of the United States, 1996, CDROM, U.S. Census Bureau, 1997, esp. table 374, spreadsheet version. Supplemental data from Historical Statistics of the United States: Colonial Times to 1970, U.S. Census Bureau, 1976.