The hundredth monkey effect is a supposed phenomenon in which a learned behavior spreads instantaneously from one group of monkeys to all related monkeys once a critical number is reached. By generalization it means the instantaneous, paranormal spreading of an idea or ability to the remainder of a population once a certain portion of that population has heard of the new idea or learned the new ability.

In 1985, Elaine Myers re-examined the original published research in “The Hundredth Monkey Revisited” in the journal In Context. In her review she found that the original research reports by the Japan Monkey Center in Vol. 2, 5, and 6 of the journal Primates are insufficient to support Watson’s story. In short, she is suspicious of the existence of a hundredth monkey phenomenon; the published articles describe how the sweet potato washing behavior gradually spread through the monkey troupe and became part of the set of learned behaviors of young monkeys, but she doesn’t agree that it can serve as an evidence for the existence of a critical number at which the idea suddenly spread to other islands.

However, the story as told by Watson and Keyes is popular among New Age authors and personal growth gurus and has become an urban legend and part of New Age mythology. Also, Rupert Sheldrake has cited that a phenomenon like the hundredth monkey effect would be an evidence of Morphic fields bringing about non-local effects in consciousness and learning. As a result, the story has also become a favorite target of the Committee for the Scientific Investigation of Claims of the Paranormal and was used as the title essay in The Hundredth Monkey: And Other Paradigms of the Paranormal published by them in 1990.

In his book Why People Believe Weird Things, Michael Shermer explains how the urban legend started, was popularised, and has been discredited.