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Between 1952 and 1953,[i] primatologists conducted a behavioral study of a troop of Macaca fuscata (Japanese macaque or Snow monkeys) on the island of Kōjima. The researchers would supply these troops with such foods as sweet potatoes and wheat in open areas, often on beaches.[1] An unanticipated byproduct of the study was that the scientists witnessed several innovative evolutionary behavioral changes by the troop, two of which were orchestrated by one young female, and the others by her sibling or contemporaries. The account of only one of these behavioral changes spread into a phenomenon (i.e., the 'hundredth monkey effect'), which Watson would then loosely publish as a story.[1]

According to Watson, the scientists observed that some of the monkeys learned to wash sweet potatoes, initially through an 18-month-old female member (named "Imo" by the researchers) of the troop in 1953. Imo discovered that sand and grit could be removed from the potatoes by washing them in a stream or in the ocean. Gradually, this new potato-washing habit spread through the troop—in the usual fashion, through observation and repetition. (Unlike most food customs, this behavior was learned by the older generation of monkeys from younger ones.)[1]

This behavior spread up until 1958, according to Watson, when a sort of group consciousness had suddenly developed among the monkeys, as a result of one last monkey learning potato washing by conventional means (rather than the one-monkey-at-a-time method prior). Watson concluded that the researchers observed that, once a critical number of monkeys was reached—i.e., the hundredth monkey—this previously learned behavior instantly spread across the water to monkeys on nearby islands.[1][6]

Watson first published the story in a foreword to Lawrence Blair's Rhythms of Vision (1975);[6] the story then spread with the appearance of Watson's 1979 book Lifetide: The Biology of the Unconscious.

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