Between 1952 and 1953, primatologists conducted a behavioral study of a troop of Macaca fuskata (Japanese snow monkeys) on the island of Kojima The researchers would supply these troops with such foods as sweet potatoes and wheat in open areas, often on beaches. 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.
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.)
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 (symbolically the 100th 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 (critical mass)—i.e., the hundredth monkey—this previously learned behavior instantly spread to all the monkeys in the group almost overnight and across the water to monkeys on nearby islands and even noticed the monkeys on the mainland also began to wash there potatoes in a shared consciousness. This amazed researchers that this learned behaviors quickly became subconscious instinct that can be witnessed to this day.
Watson first published the story in a forward to Lawrence Blair's Rhythms of Vision (1975); the story then spread with the appearance of Watson's 1979 book