Strauss and Howe argue that the last five centuries of Anglo-American history can be explained by the existence of four generational archetypes that repeat sequentially in a fixed pattern every 80-100 years, the length of a long human life, or what the ancients called a “saeculum.” These generational archetypes are: Prophet, Nomad, Hero, and Artist. Each generation consists of those born during a roughly 20 year period.
A generation reaches it apex of influence when it moves into midlife and begins to take leadership positions of power within society. Thus every 20 years as a new generation fills the midlife rung of the age ladder, and the generation that previously occupied that rung moves into less influential elderhood, the mood of the culture shifts. As each generation type is born, matures, comes to influence in the culture, and then declines and dies, it plays a role in propeling society through a cycle of growth, maturation, entropy, destruction, and then regrowth. Just as in nature, this cycle of death and rebirth is necessary to maintain the health of the ecosystem or society.
The theory will be easiest to grasp and keep track of if you think in terms of who the generational types were/are during our most recent saeculum as we go along:
Most Recent Generations:
Heroes: G.I. Generation (born 1901-1924)
Artists: Silent Generation (born 1925–1942)
Prophets: Baby Boom Generation (born 1943-1960)
Nomads: Generation X (born 1961-1981)
Next Heroes: Millennial Generation (born 1982-2004)