To the Brain, the Number Seven is Special

There are seven days of the weeks.   Telephone numbers originally started with seven digits.  There are seven whole musical notes in a octave.  Seven continents.  Seven seas.  Seven wonders of the world.  Seven deadly sins.  Seven stars in the big dipper constellation.  Why did the convenience store name themselves "7-Eleven"?  Why have a soft-drink named "7-up"? And many others occurrences of the number seven as listed on Wikipedia.

Surely some of these seven's are pure coincidences.  And some are just pure fun --  such as seven ingredients in a McDonald's Big Mac as cited in the Big Mag song that goes "Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun" ^[ref] Assume both patties as one ingredient.  And the special sauce is one ingredient.

But perhaps there is a psychological basis in which the number seven comes up so often is because the human brain can hold about seven independently discrete bits of information simultaneously in short-term memory.

An article titled "In the Brain, Seven Is a Magic Number" cites a paper by Mikhail Rabinovich and Christian Bick that may explain this short-term memory limit as being due to neuronal firings.

According to Wikipedia, "estimates of short-term memory capacity limits vary from about 4 to about 9 items". Because of this variance among people, this capacity is often cited in literature as "7 plus or minus 2".^[ref]

In fact, Jonah Lehrer in a talk that he gave to the Commwealth Club also mentioned this "7 plus or minus 2" limit of the pre-frontal cortex. He explains and how this limitation can sometimes make the brain makes wrong decision when it is asked to reason and analyze too many variables at once.

This "seven plus or minus two" phrase originated from a paper published by George Miller of Harvard University in Vol 63 of "The Psychological Review" dated March 1956 that is titled "The Magical Number Seven, Plus Or Minus Two: Some Limits on Our Capacity for Processing Information".

Although this is an academic paper with charts and results of experiments, Miller starts this paper with a lead-in that sounds almost like an adventure novel. He writes, "My problem is that I have been persecuted by an integer. For seven years this number has followed me around, has intruded in my most private data, and has assaulted me from the pages of our most public journals. This number assumes a variety of disguises, being sometimes a little larger and sometimes a little smaller than usual, but never changing so much as to be unrecognizable. The persistence with which this number plagues me is far more than a random accident. There is, to quote a famous senator, a design behind it, some pattern governing its appearances. Either there really is something unusual about the number or else I am suffering from delusions of persecution."

At the end, Miller has this to say about the number seven:

"And finally, what about the magical number seven? What about the seven wonders of the world, the seven seas, the seven deadly sins, the seven daughters of Atlas in the Pleiades, the seven ages of man, the seven levels of hell, the seven primary colors, the seven notes of the musical scale, and the seven days of the week? What about the seven-point rating scale, the seven categories for absolute judgment, the seven objects in the span of attention, and the seven digits in the span of immediate memory? For the present I propose to withhold judgment. Perhaps there is something deep and profound behind all these sevens, something just calling out for us to discover it. But I suspect that it is only a pernicious, Pythagorean coincidence."^[ref]

In his book Embracing the Wide Sky, author Daniel Tammet mentions Miller's paper saying that "short-term memory generally has a capacity of between five and nice "chunks" of information."^{[Embracing the Wide Sky page 71]}. And we can get around this limit, say to remember a 11 digit number, by "chunking" or grouping some of the digits together.

In the book Age-Proof Your Mind, Dr. Zaldy Tan says that short-term memory is "characterized by a limited storage capacity (five to seven bits of information at a time) and a short shelf life (thirty to forty-five seconds)." [page 37]

There are more than 835,000 books with the number seven in its title. This is according to Jacqueline Leo on page 9 of her book "Seven: The Number for Happiness, Love, and Success".

You can hear Neal Conan interview Jacqueline Leo in NPR program's "Talk of the Nation".

Stephen Covey and Deepak Chopra both know psychology. Perhaps is that why Covey titled his book "7 Habits of Highly Effective People" and Chopra titled his as "The Seven Spiritual Laws of Success"? Because if there are more than seven habits or more than seven spiritual laws, we would not be able to remember them.

The number 7 not only a prime number, but it is also a Mersenne prime.

When you roll a pair of dice, the value 7 is statistically mostly likely to occur. Statistically, it should come up 1 in 6 throws. You are 6 times more likely to get a seven than to get a value of 2 from the dice roll.