The Magical Number 7

Have you ever noticed that the number 7 seems, well, kind of special?  
It just seems to appear in more than its share of things.  It has been like this for very a long time.

There are 
7 wonders of the world, 7 ages of man, 7 digits in a phone number, 7 notes on the musical scale, 7 liberal arts in classical antiquity, 7 days in the week, 7 kings of Rome, 7 sages of Greece and the Bamboo Grove in China, 7 dwarfs, the 7 lucky Gods of Japan.  In Hindu weddings, the bride and groom walk around the holy fire 7 times during the ceremony for some reason.

There is also 7-eleven, 7Up, and Seven Jeans.  James Bond is 007 .   And, most importantly, DC7.  
Coincidence, you say?  Well, recent finding in neuroscience and psychology seem to suggest otherwise.   

Our brains seem to have an innate limitation regarding judgement, memory and attention somewhere just below the number 7.  People can remember, on average, a string of 7 digits and 7 random tones played on a scale before they begin to make mistakes.  When people look at a set of fewer than 7 objects or dots on a page, they count them and get the number correct; above 7 they guess at the number and usually miss.  7 is also important in memory as "chunking" methodologies often have a top-level limit of 7 distinctions.

Now consider the Golden Ratio.  That is that special number that seems to occur in nature everywhere-- the length of your navel to the ground and your overall height for instance.  It's roughly 1.618 which means that the short part is roughly .62 of the whole.  .62 is conspicuously close to the actual decimal value of the human digit span (ability to remember a string of digits).

Some scientists have suggested that, for many things, the number 7 is the whole integer just beyond the average capacity for remembering words (which is around 6), and so items listed in 7s stand as a sort of just out of reach goal to help us remember important things.  We do have 5 fingers after all, although it is the product of a recessive gene that gives us five instead of 6.  Remembering or counting five things is easy for us.  As is 6.  

So perhaps 7 is important for lists and notables because it is the first number that is just beyond our natural capacity, and maybe that natural capacity is the result of a biological design which somehow adheres to the Golden Ratio.

Whatever the reason, there is no denying that there is something special about the number 7.