regardez s'il vous plait le following graph.
http://piketty.pse.ens.fr/files/capital21c/en/pdf/F8.7.pdf
The curves symbolize the share of national income of the wealthiest 10% of the population. You will note that in the early decades of the 20th century, there was an enormous amount of income inequality, and it was rising in spite of WWI and the Great Depression. During WWII, the inequality plummeted--we simply could not afford to pamper the rich any more and still mobilize for total war.
After 1945, the inequality stabilized at about 32% (the share of national income the top 10% kept for itself). This explains the relative prosperity of the postwar era.
But look what happens around 1980, including the role of capital gains. Guess what happened in 1980?