From 1939 to 1973, during the 24 years from the start of WW2 to the first oil crisis the US’s average income grew by $30,500. 72% of this economic growth in incomes went to the poorest 90%, and 28 % went to the richest 10%.
Over the following 24 years, between 1974 and 2008, the US’s average income grew by $11,000 – average income of the poorest 90% declined, and all the growth went to the richest 10%, mostly to the richest 1%.
Why? One plausible explanation points to the "winner-take-all" nature of American politics, and the use of wealth to purchase political power, which in turn is used to protect and transfer wealth to the top one percent.
A tip of the hat to Resilience Science and economist Emmanuel Saez.