Hundredth monkey effect

The hundredth monkey effect is a claim that a behavior spreads abruptly through a population once enough individuals adopt it, sometimes attributed to collective consciousness. Its popular form grew from accounts of Japanese macaques learning to wash sweet potatoes.

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Japanese macaque: A macaque species native to Japan, known for living in groups across varied climates. The original accounts concerned sweet-potato washing by macaques on Kōjima.
Social learning: Learning that occurs by observing or interacting with other individuals. It explains how the macaque behavior could spread without any population-wide mental threshold.
Imo: A young female Japanese macaque on Kōjima credited with beginning sweet-potato washing in her troop. Accounts identify her as the first animal observed washing the food.
Morphic resonance: Rupert Sheldrake’s hypothesis that natural systems inherit patterns through a nonmaterial field of collective memory. The hundredth monkey story has often been linked to this speculative explanation of shared behavior.
Kōjima: A small island in Miyazaki Prefecture, Japan, where Japanese macaques have been studied since the mid-twentieth century. Researchers observed the troop behavior that later inspired the effect’s name.
Cultural transmission: The transfer of learned behaviors, knowledge, or practices between individuals or generations. The effect’s observable premise is a form of cultural transmission, even if its proposed mechanism is disputed.
Japanese macaque potato-washing study: A series of primatological observations of Japanese macaques learning to wash sweet potatoes on Kōjima. The research records gradual adoption rather than a documented sudden threshold event.
Rupert Sheldrake: An English author and former biochemist known for proposing morphic resonance. His ideas helped connect the anecdote to theories of collective memory.
Imo washing: A learned behavior in which Japanese macaques wash sweet potatoes before eating them. This gradual, socially transmitted behavior became the anecdote’s empirical starting point.
Diffusion of innovations: A theory describing how new ideas and practices spread through a population over time. Its adoption curves offer a conventional account of apparent shifts in group behavior.