The term "Hundredth Monkey Phenomenon" refers to the instantaneous spread of behavior to a whole population when a critical number of individuals in that population adopt that behavior. It gets its name from an incident involving macaque monkeys on Koshima Island, Japan. In the 1950's, primate behavior researchers began giving the monkeys sweet potatoes. Most of the macaques ate the potatoes dirty, but some began rinsing them in sea water first. The habit was slow to catch on, but as the story goes, when the number who rinsed their potatoes reached 100, suddenly every macaque on Koshima adopted the practice. The incident has been used to encourage the spread of desirable behavior, with the claim that if enough people engage in the behavior, then it will instantaneously spread to everyone else.
Alas for the hopes of promoters of good behavior, the historical record shows that the story is not true. The habit of potato washing didn't instantly spread throughout the macaques when a crucial number was reached: it spread gradually, by the normal means of teaching and imitation. There is no shortcut to spreading desirable practices by getting a critical number to use the practice; each person must learn the behavior. The story of the "Hundredth Monkey" started as a parable to illustrate the notion that a behavior may extend instantaneously throughout a population once a critical number of members using the behavior is reached. Neither the story nor the idea is supported by the evidence.
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