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The Curious Case of Hundredth Monkey Effect and How Ideas Actually Seem to Spread

The Curious Case of Hundredth Monkey Effect and How Ideas Actually Seem to Spread

During this weekend series, I’ve primarily stuck to phenomena that have held up to empirical research or at least heavily been supported by clinical practice.

In today’s article, however, I’d like to talk about a completely hypothetical phenomenon called hundredth monkey effect.

The Hundred Monkey Effect

What is hundredth monkey effect? It’s a hypothetical phenomenon explaining how ideas or behaviors spread from just a few individuals to a much wider population. Hundredth monkey effect basically states that there is a point at which an isolated social phenomenon reaches critical mass and thereby reaches a threshold where it becomes inevitable that it will become incredibly popular.

The idea was first born from a small study in the 50s conducted on Japanese monkeys, in which the behavior of washing sweet potatoes spread unexpectedly among the larger group. This was a completely incidental finding — the study was focused on something else. However, the story about the study and the findings were summarized in the 70s in the introduction to a guide on mysticism (yes, really).

It’s worth noting that in this introduction the scientists in the Japanese monkey study went unnamed. Still, the writer of this introduction claimed that the sweet potato washing had spread to the larger group once it had reached critical mass — a point which he deemed the hundredth monkey.

During the 80s, the term the hundredth monkey was further popularized in a book warning against the dangers of nuclear power by Ken Keyes, Jr., called The Hundredth Monkey. Again the author didn’t provide substantive verification of the original study, nor did he support additional claims he made about the behavior spreading to islands adjacent to where the original study was claimed to have taken place.

Is Hundredth Monkey Effect True?

So is any of the story true?  Was the hundredth monkey effect found in a study of Japanese monkeys conducted in the 50s?

Yes (some of the story is true) and no (hundredth monkey effect was not found).

There was a study conducted in the 50s by  on monkeys by Japanese researchers in which a lone monkey was first observed to wash her sweet potatoes, and later other monkeys did come to do it as well. However, a hundredth monkey effect was not observed. The spread of the behavior did not seem to be related simply to a critical mass or number of monkeys learning the behavior. Instead the following findings were observed:

  1. Young monkeys taught their peers and their close family members. Those monkeys in turn would also teach the behavior to their own peers and close family members.
  2. Monkeys who were too advanced in age would not adopt the behavior, regardless of how many peers or contemporaries attempted to teach them.
  3. Children of monkeys already doing the behavior automatically imitated it and didn’t need to be explicitly taught to do it.
  4. The initial monkey who washed sweet potatoes experimented with washing wheat. Basically, this one monkey emerged as a repeat innovator.
  5.  The innovative monkey’s sibling began to experiment in other ways, starting a trend where monkeys who had previously been afraid of getting in the water were now splashing and playing in the ocean (the process of washing the wheat required them to get partly wet and seems to have spurred on this phenomenon).

In any event, social relationships between members seemed key as did the age of the monkeys (very much keeping with the adage “you can’t teach an old dog new tricks”).

There did not seem to be a special mathematical ratio responsible for the proliferation of the behavior.

Enough with Monkeys, How About Humans?

I’m not convinced that all of this study easily applies to human interactions. Most glaringly, while personality does tend to firm up at a few key points in human development (by statistical average, there are big solidification points at 30 and 50, according to developmental personality research, having do with frontal lobe development and cognitive changes due to aging), making a person’s overall behaviors more ingrained, there doesn’t seem to be a firm cut-off point where older adults simply will not learn anything new.

However, the rest of it does ring true when looking at the way that information and ideas tend to spread through online social networks. You do see people’s proximity to others dictating a lot of what they know, believe, and do.


This post is part of an ongoing Poly Land feature called Psyched for the Weekend, in which I geek out with brief takes about some of my favorite psychological studies and concepts. For the entire series, please see this link.

Featured Image: CC 0 – Pixabay