“There is a mysterious cycle in human events,” said Franklin Delano Roosevelt, accepting the Democratic nomination for president in Philadelphia in 1936. “To some generations much is given. Of others much is expected. This generation of Americans has a rendezvous with destiny.”

In the 20th century, many sociologists and historians flirted with the idea that generational changes could explain U.S. politics. The historians Arthur Schlesinger Sr. and Jr. wrote about “cycles of American history,” arguing that, as the generations turn, American politics rotates inexorably between liberal and conservative consensus. More recently, a new generational scheme has come into vogue. William Strauss and Neil Howe’s theory of the “fourth turning” predicts a crisis and a major political realignment every 80 to 90 years. (Strauss and Howe were briefly in the spotlight in 2016 after Steve Bannon praised their work.)

We are skeptical about cyclical theories of history. We are also aware of the slipperiness of generations as categories for political analysis. As Karl Mannheim pointed out more than 90 years ago, a generation is defined not solely by its birth years but also by the principal historical experience its members shared in their youth, whatever that might be. Nevertheless, we do believe that a generational division is growing in American politics that could prove more important than the cleavages of race and class, which are the more traditional focuses of political analysis.

Representative Alexandria Ocasio-Cortez is often described as a radical, but the data show that her views are close to the median for her generation. The Millennials and Generation Z—that is, Americans aged 18 to 38—are generations to whom little has been given, and of whom much is expected. Young Americans are burdened by student loans and credit-card debt. They face stagnant real wages and few opportunities to build a nest egg. Millennials’ early working lives were blighted by the financial crisis and the sluggish growth that followed. In later life, absent major changes in fiscal policy, they seem unlikely to enjoy the same kind of entitlements enjoyed by current retirees.