Of all the infinitely many numbers in existence, the number seven appears to be humanity's favorite. Why?

In 2011, London-based writer Alex Bellos – author of popular mathematics books Here's Looking At Euclid and the recently published The Grapes of Math conducted an online survey in search of the world's favorite number. The survey polled upwards of 30,000 people, and more than a thousand individual favorite numbers were submitted. Of those submissions, nearly half were for numbers between one and ten. It wasn't the most scientifically rigorous survey, Bellos admits – but it's hard to ignore his results, which place seven in a clear position of dominance.

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Among the people Bellos surveyed, 7.5% percent of them voted for the number three, which finished in second place. Third place went to the number eight (capturing 6.7% of the vote); fourth place to the number four (5.6%); and fifth place to the number five (5.1%). Places six through 10 (numbers 13, 9, 6, 2, and 11, in order) each seized somewhere between 5 and 2.9% of the total vote. But way out in front, dominating the pack, was the number seven. Of the more than 30,000 people polled, nearly 9.7% of them identified seven as their favorite number. "It wasn't even close," Bellos tells io9.

To gain some insight into what people find so attractive about seven, Bellos asked survey participants to not only nominate their favorite numbers, but explain their affinity for them. The explanations people provided were manifold. Here's a representative sampling (these being for seven, specifically):

  • "Seven is the number of stellar objects in the solar system. Seven is the number of chakras. Seven is Sunday! Seven is the calling code for Russia. Seven just feels magical!"
  • "Nice shape, simple line with vertical and horizontal interest ... a number that is growing up. It's a bit awkward; it can't be equally divided and won't bend to the rules so easily!"
  • "People, don't usually tend to pick 7, and I like to be different." [Ed.: Bellos tells me he used to appear on radio programs to talk about his survey, and that the discussion on these programs would inevitably turn to the popularity of the number seven. He says that, when the survey was still live, he could count on seven being dramatically underrepresented in the next day's responses. "People want to be unique," he says.]

These responses, and others like them, make two things abundantly clear. The first is that our feelings about the number seven – or any number for that matter – are intricately entangled with things like culture, language, and visual representations*. And the second is that the answer to what people find so attractive about the number seven can be pretty difficult to nail down.

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At Psychology Today, University of Massachusetts psychologist Susan Krauss Whitborne lists seven possible explanations, ranging from the mystical :

The religious and spiritual associations to the number 7 go back through the millennia, ranging from the 7 deadly sins to seventh heaven.

To the psychological:

...psychologist George Miller observed many years ago that our short-term memory remembers in units of 7 plus or minus 2. You can remember an infinite list of words, tasks, or facts if you organize it into 5 to 9 (but ideally 7) chunks.

According to Bellos, one of the most popular explanations has to do with the number's prevalence in the natural world. In antiquity, for example, there were seven classical "planets" visible in the night sky, namely Mercury, Venus, Mars, Jupiter, Saturn, and, because they were not stationary in the sky (the word "planet" is derived from the Greek planētēs, meaning "wanderer"), the Sun and Moon. But this, says Bellos, is probably a red herring, a case of post-rationalization. Like many instances of seven's commonness in the natural world, the visibility of seven moving heavenly bodies, he says, is probably just a coincidence.

You'll find this coincidence everywhere, if you look for it. There are seven days in a week. Seven colors in the rainbow. Seven continents. Seven seas. But how many of these coincidences are truly accidental? Which of them did we create – (the notion that there are just seven wonders in the world is just silly; and the seven-day week isn't a natural construct, it's a human one), and which of them exist independently of our actions and observations, perhaps influencing our subconscious appreciation for the number seven?

You see how this line of inquiry can lead pretty quickly to the kinds of questions one might raise between bong hits. Fortunately, Bellos is here to keep us grounded. You want an explanation for seven's popularity? He's got one.

"I think that arithmetic uniqueness is the best explanation for the success for seven," says Bellos. What's more, he says, that uniqueness is immune to the warp that time inflicts on the lenses through which we view the world. Language, culture, numerals, writing – all of these, says Bellos, change with time. But what hasn't changed, he says, is the arithmetical structure of seven.

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Seven is the only number, among those we can count on our hands, that cannot be divided or multiplied within the group. One, two, three, four, and five can all be doubled, to give two, four, six, eight, and ten. Six, eight, and ten can all be halved to give three, four, and five; and nine is divisible by three. But seven? Seven is special. "It's unique," Bellos explains in the video below. "It's a loner. The outsider. And humans interpret its arithmetical property in cultural ways. By associating seven with a group of things, you kind of make them special, too."

Bellos refers me to the results of a study published in a 1967 issue of the Journal of Experimental Psychology, titled "The Predominance of Seven and the Apparent Spontaneity of Numerical Choices." In the study, Yale psychologists Michael Kubovy and Joseph Psotka asked test subjects to think of a number between one and ten, and found that most people settled on seven. Why? According to Bellos, it has to do with the numbers perceived uniqueness, and what Kubovy and Psotka identify as a desire to "comply with the request for a spontaneous response."

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What Kubovy and Psotka propose is that test subjects are, perhaps subconsciously, running through precisely what the animation at the top of this post illustrates. You'd never say 1 or 10 because, as end points, they're not arbitrary enough. You don't say five, because it's right in the middle; it doesn't feel random. Two, four, six and eight all feel a bit too ordered, so they're eliminated. "Your brain, without realizing it, is doing these calculations," says Bellos, and "the one that feels most arbitrary is the number seven."

Or, as Kubovy and Psotka put it, seven is "in the unique position of being, as it were, the 'oddest' digit."


If you're into this sort of thing, I highly recommend checking out Bellos' new book, The Grapes of Math (Alex Through the Looking-Glass in the UK), in which he discusses his survey in greater detail, the cultural significance of seven, and other issues about our psychological responses to numbers

* Bellos says that if you confined his survey to China, for example, you would probably see the number eight in pole position, but he predicts that you would see virtually no fours. "The number four is unlucky in China, because it sounds like the word for 'death,'" he says. Conversely, the word for "eight" sounds like the word for "prosper" or "wealth." Another consideration, regarding visual representations: Do people relate to the abstract concept of the number seven differently from the symbolic, numerical representation of that number – the simple glyph, 7, "with vertical and horizontal interest," that many of us see and think "ah, yes, seven!"?

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