Today is Friday the 13 th . Again. Cultures all over the world consider this an unlucky date, despite the fact that there’s no logical or scientific reason to support this notion. So why do people believe Friday the 13 th is unlucky? And why won’t this notion go away?

Today is Friday the 13th. That’s bad luck for a February, what with it having fewer days as it is, but it’s not the first time this has happened and it won’t be the last. There are three Friday the 13ths occurring this year. Many won’t like hearing this, as Friday the 13th is strongly associated with bad luck. There’s even a horror franchise named for it. Admittedly, being hacked up by a machete-wielding serial killer would certainly count as “bad luck”, but then that would be true whatever day it happened on.

The origins of the superstition around Friday the 13th are a bit muddy, but some point to the efforts of a high-profile group actually set up to challenge superstition, followed by a 1907 book about how Friday the 13th “misfortune” is used to destabilise the stock market. It’s called “Friday the Thirteenth” and features stockbrokers, not hockey-mask wearing, machete wielding murderers (although which is worse is debatable).

There’s an actual phobia of Friday the 13th, paraskevidekatriaphobia. But a “phobia” is an irrational fear of something. This implies that most people know, at least on some level that Friday the 13th is no more unlucky than any other day. And yet the belief persists on a major scale. What’s going on there?

Friday’s are sometimes thought of as unlucky. Many people look forward to Friday, but not everyone. If nothing bad has happened all week, maybe it feels more “likely” to occur on Friday? Fridays may also feel more rushed and hectic if time sensitive tasks have to be completed before a weekend.

But it’s the number 13 that’s widely considered to be unlucky (for some). It has its own phobia as well. But it’s only a number. There are loads of numbers, an uncountable amount (ironically), so why single a specific one out as unlucky when there are so many?

Firstly, 13 is an odd number. In his book “Alex Through The Looking Glass”, co-blogger Alex Bellos describes some of the theories as to why odd numbers seem more “salient” to people than even ones. There’s a surprising amount of psychology underlying people’s perception of numbers, although given how long numbers have been around maybe it isn’t surprising. One argument is that even numbers are easier to manipulate (by dividing or multiplying) than odd ones, so odd ones require more brain power to utilise, so they “resonate” more. This doesn’t necessarily mean it’s a good thing, as something requiring more biological resources can easily be off-putting, as anyone who’s ever had to persuade themselves to go to the gym will attest to.

There’s also the view that even numbers are neater and more “complete” than odd ones, which had associations with spirituality back in more historical times.

We can also consider the effect of familiarity on our perceptions. At simple levels, familiarity can make something more pleasing (which ties in with the using-less-resources observation from earlier, as we don’t need as much brain power to process an already familiar stimulus). And a particularly familiar number for people is 12; 12 months of the year, 12 hours on a clock, 12 days of Christmas, 12 apostles, 12 inches in a foot, the Dirty Dozen, a dozen eggs, etc. 13 being so close to 12 but “not quite” makes it jarring. Also, it’s worth considering the religious significance of 12 which 13 could be said to “violate”, hence it’s bad. It’s a very old superstition, after all.

13 is also arguably the first “unfamiliar” number we encounter. In school we learn our multiplication tables, and these usually stop at the 12 times table, probably due to the mathematical usefulness of the duodecimal (base 12) system. So we aren’t taught about 13. It doesn’t get its own times table, and it doesn’t feature in any of the others, what with it being a Prime number. It’s not the smallest prime number, but it’s the smallest one without its own dedicated times table, so is cognitively “weird”. But given it’s a low number overall in the grand scheme of things, 13 likely occurs often enough to be noticed, given it this seemingly-paradoxical mix of familiarity and mystery, just right for association with bad luck and portents in a less-than-rational world.

13 is also 1 then 3, which is neither a repetition (like 11) or familiar order (like 12, 1 followed by 2). This could also be jarring. If you look even more closely, 13 does look a lot like a capital “B”. B stands for Bad luck! And Bombs. And Botulism. And Bryan Adams. All of these things are worrying. It also stands for Breasts. And who has breasts? Women! And what are women? Oppressed! Breast even rhymes with oppressed, and that’s hardly lucky, so it all adds up!

You probably noticed that the previous paragraphs crossed the line from seemingly plausible to mad ranting at some point, but here’s the interesting part; at what point? Many would consider the very concept of psychological preference for certain numbers to be nonsense, others see significant patterns in everything so could believe even the obvious joke bits.

Therein lies the possible cause of the Friday the 13th persistence; cherry picking. When someone has a belief or notion that they think is true, it’s very easy to find evidence to support it, as long as you interpret it in useful ways and ignore anything that contradicts it.

The idea that Friday the 13th is unlucky is rife in our society, so it’s bound to be picked up by people. And once you have this idea, normal occurrences become twisted or more important. Your car breaks down on Tuesday the 23rd? That’s because you didn’t get it serviced when you should have. It breaks down on Friday the 13th? That’s because it’s Friday the 13th! People being more watchful for unfortunate things happening are far more likely to find them, and thus the idea persists and spreads, especially when mainstream pieces like this keep bringing it up.

People believing in ridiculous or superstitious things isn’t exactly uncommon. Despite our scientific advancements, horoscopes are as popular as ever, alternative medicine, conspiracy theories, and so on. With such far-fetched notions being passed around as fact, a particular day being unlucky seems quite mild in comparison. “Blue Monday” is still genuinely thought to be the most depressing day of the year across the world, and that’s a purely twenty first century phenomenon. Friday the 13th is far more culturally entrenched, and probably will be for some time to come.

Dean Burnett is on Twitter and likes to keep an even number of followers, just to be safe. @garwboy