I suggest looking into the work of Peter Turchin.

Turchin, following up on many precious writers who’ve noticed certain cycles or repeating themes in history, has posited that there is an ebb-flow to history that is about 80 years, about the lifespan of a human.

What drives this flow, in this theory, is majority one force. The rise in birth rates during good times, followed by the trouble of environments and economies to absorb the new individuals, eventually leading to some sort of new adjustment among all players.