America’s upper middle class has doubled in size since 1979, a new study published Tuesday finds, representing a major step forward in living standards for U.S. workers but also a possible explanation for the discontent of the majority of people who have seen slower income growth.

In an analysis published by the Urban Institute Tuesday, scholar Stephen Rose examines Census data to find that the upper middle class has grown from under 13 percent of the U.S. population in 1979 to nearly 30 percent in 2014, siphoning off families from the lower classes. Furthermore, the upper middle class now earns more than half of all income, up from 30 percent.