According to a number of measures, income has grown more slowly for middle-class households since the 1970s than for those with higher incomes, resulting in wider income inequality. The kind of generational improvements in living standards that were long the hallmark of the American middle class have steadily diminished.
In the 1950s, better than 80 percent of children born in middle-class households grew up to outearn their parents, but more recently only around half do. One factor in this decline is the increase in income inequality I just noted, and another is slower productivity growth.