Generations: The Real Differences Between Gen Z, Millennials, Gen X, Boomers, and Silents—and What They Mean for America's Future

by Jean M. Twenge

In their 1991 book, Generations, Strauss and Howe argued that major events caused generations to cycle through four different types (Idealist, Reactive, Civic, and Adaptive), with each type suited for their age during the event.