Did middle-class incomes really decouple from overall economic growth in the mid-'70s? If you look at median family income vs. GDP per capita, the answer is yes. From 1950 through 1975, both grew at about the same rate. After that, median family income grew quite a bit more slowly than GDP per capita.
But wait! You need to make sure to calculate inflation the same way for both measures. And maybe GDP per capita is a bad measure. Plus you need to account for health insurance and other benefits when you calculate median income. And the number of people per household has changed over time. These are all legitimate issues. So Lane Kenworthy redrew the chart to compare apples to apples: median household income vs. average household income. Median income shows only the movement of households that are smack in the middle of the middle class, while average income is similar to overall economic growth since it depends on total national income.
In the chart below, the black lines are the original comparison. The red lines are the new comparison. As you can see, there's really not much difference. "Decoupling," say Kenworthy, "is real and sizable." The rich really are hoovering up a much bigger share of national income than they used to. The only thing left to argue about is why, not whether.