The 1960 United States census, conducted by the Census Bureau, determined the resident population of the United States to be 179,323,175, an increase of 19 percent over the 151,325,798 persons enumerated during the 1950 census. This was the first census in which all states recorded a population of over 200,000. This census's data determined the electoral votes for the 1964 and 1968 presidential elections. This was also the last census in which New York was the most populous state.
Detroit lost 220,000 residents in the 1950s, which remains the record for the largest loss of population between two censuses.
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Text from the Wikipedia article “1960 United States Census”, available under CC BY-SA 4.0.