The Fourth Turning Is Here: What the Seasons of History Tell Us about How and When This Crisis Will End

by Neil Howe

An estimated two-thirds of all mammalian species exhibit multi-year cycles of population expansion and contraction. These range from roughly four years for lemmings and voles to ten years for snowshoe hares to thirty-eight years for moose. Most of these cycles appear to be unrelated to climate, predators, or anything else in the environment. More intriguingly, many are linked to a matching behavioral pattern—for example, cycles of aggression, herding, migration, mating, and stress. In effect, these animals act differently depending on when, during this repeating calendar, they are born. While biologists aren’t sure what drives these cycles, some have speculated that they may be triggered by periodic changes in the animals’ production of hormones and neurotransmitters and that these changes may be synchronized by pheromones and behavioral cues. No one has any idea whether this triggering does happen or could happen in humans.