The story of the "Hundredth Monkey Effect" apparently originated with Lyall Watson in his 1979 book Lifetide. In it he claimed to describe the observations of scientists studying macaques (a type of monkey) on the Japanese island of Koshima in 1952. Some of these monkeys learned to wash sweet potatoes, and gradually this new behavior spread through the younger generation of monkeys—in the usual fashion, through observation and repetition. However, Watson claimed that the researchers observed that once a critical number of monkeys was reached—the so-called hundredth monkey—this previously learned behaviour instantly spread across the water to monkeys on nearby islands.

This story was further popularized by Ken Keyes Jr with the publication of his book The Hundredth Monkey. Keyes presented the "Hundredth Monkey Effect" story as an inspirational parable, applying it to human society and the effecting of (positive) change therein. Since then, the story has become widely accepted as fact, and has even appeared in books written by some educators.

The content of the book by Keyes was a substantive treatise on the effects of nuclear
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war on the planet and the devastation caused thereon.
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An example of the effect Keyes was seeking is captured in some degree by a two-month prospective experiment and study to reduce violent crime in Washington DC in 1993. Based on the results of the experiment, the steady state gain (long-term effect) associated with a permanent group of 4,000 participants in the Transcendental Meditation and TM-Sidhi programs was calculated as a 48% reduction in violent crimes in the District of Columbia. More can be found here: http://www.istpp.org/crime_prevention/, and many other places on the web. It is seen by many as a prime example of confusing correlation with causation.

Rupert Sheldrake has cited the "Hundredth Monkey Effect" as evidence of morphogenetic fields bringing about non-local effects in consciousness and learning.