From an article by the Center on Budget and Policy Priorities:
Piketty and Saez’s unique data series on income inequality, based on IRS files, is particularly valuable because it provides detailed information on income gains at the top of the income scale and extends back to 1913. These data show that in the past decade, income concentration has reached levels last seen over 80 years ago (see Figure 2).
The uneven distribution of economic gains in recent years continues a longer-term trend that began in the late 1970s. In the generation following World War II, robust economic gains were shared widely, with the incomes of the bottom 90 percent actually increasing more rapidly in percentage terms, on average, than the incomes of the top 1 percent. But since the late 1970s, the incomes of the bottom 90 percent of households have essentially stagnated while the incomes of the top 1 percent have soared. (See Figure 3.)