Today is Friday the 13th. For no logical reason, people around the world consider it to be a profoundly unlucky day. This is a superstition with no clear origin, although it certainly seems to be widespread.

But is there any scientific truth to this story? Ruling out any mystical forces at work, is Friday the 13th actually as unlucky as people claim? Could there be a self-fulfilling prophecy where people experience more bad luck because they expect to, or is it in fact the opposite, where people behave particularly cautiously in order to avoid any misfortune?

Numbers Game

First things first: Is there any statistical evidence to suggest that Friday the 13th is actually a misfortune-prone day?

There are some statistical studies that do reference the effect Friday the 13th will have on accident rates. A few have looked specifically at car accidents: a 2002 Finnish study shows that up to 38% of traffic deaths involving women on this day may be attributable to the increased anxiety associated with the "unlucky" date; conversely, a 2008 Dutch study shows that there were less accidents overall on Friday the 13th. In 1993, a British Medical Journal study investigating the phenomenon – or lack thereof – appears to agree with the Finnish study, stating that there may be a 52% increase in traffic accidents on that day; the authors of the study actually recommend staying at home that day.

However, the sample sizes of these very limited studies are incredibly small, and attributing accidents to a difficult to define psychological effect is incredibly difficult to do accurately. “It’s really hard to find any actual verifiable data of it,” Dean Burnett, a doctor of neuroscience, lecturer and author, told IFLScience. “People report incidents every day of the year. How do you ascribe an incident to be in bad luck rather than incompetence?

“The sample will be affected by people attempting to be especially more careful on a day they 'know' to be unlucky. And besides, what counts as unlucky?” After all, losing your house to a hurricane and accidentally dropping your phone into a toilet are both unlucky, but the former is inarguably far worse.

So without any reliable statistical evidence, how did the rumor ever begin?

Superstition, myth, and hype

The number 13 has historically been viewed as unlucky, and superstitions of odd numbers go way back. Burnett also noted that our brains prefer even numbers as they are easier to do mental calculations with, whereas odd numbers require more thought.

Thirteen is also the first “unfamiliar” number we encounter, distinct from 12 hours on a clock face, 12 months in a year, and our multiplication practice, which ends at 12 as youngsters. We are innately more comfortable with familiarity – this is a psychological phenomenon known as the mere-exposure effect.

It may be a mix of religion and myth. There have been suggestions it is related to the Christian story of Jesus dying on a Friday following the Last Supper, which hosted 13 people, where he was betrayed. In the Norse legends, Loki is the 13th guest at a dinner set for 12 gods honoring Baldur, whose death he is responsible for. 

As for the supposedly unlucky combination of Friday and the number 13? “I can’t say for certain, there’s no actual basis in reality for why this is the case,” Burnett continued. “Perhaps people are more careless on a Friday; people are still working, but they’re less fresh, less focused.” So accidents may occur more frequently on these days, and the association with the number 13 merely exacerbates people’s perception of how accident-prone they are at the time.

One of the earliest events associated with it being unlucky is the execution of hundreds of Knights Templar, a military group that sought to defend the Holy Land, by the French in 1307. Author and American businessman Thomas Lawson encouraged the myth when his book Friday, the Thirteenth – about a stockbroker who chooses that day to bring down Wall Street –  was published in 1907.

False Positives

There is at least one but no more than three Friday the 13ths every year. That's roughly 172 Friday the 13ths every century, so that’s quite a lot of accumulated bad luck, which is bad news for those suffering from paraskevidekatriaphobia – the debilitating fear of this day. As Burnett points out, however, this is indeed a phobia, an irrational fear, which suggests that people know that their anxiety is factually unfounded and certainly misplaced. But then irrational fears are closely related to superstitious beliefs, which essentially involve seeing patterns that aren’t actually there. Humans, over their long evolutionary history, have developed a keen sense of pattern detection, but unfortunately sometimes this trips us up.

Say you're walking through a forest and hear a rustling nearby. Could this be a dangerous predator, or perhaps just the wind? Both are likely, but only one will be true at any one time, so we tend to react based on which interpretation we instinctively choose. We are, however, more likely to survive in the long term if every time we hear rustling, we assume that it’s a dangerous predator. In most cases, we will leap out of the way, only to find it’s nothing more than the wind. In some cases, we’ll be right, and we’ll survive an attack.

So, naturally, we are evolutionarily hardwired to believe that there are threats – or merely just patterns – out there in the world even if most of the time there aren’t. It’s a survival instinct, and we all experience these “false positives” from time to time. So perhaps Friday the 13th’s reputation comes from anecdotal false positives across the world, building up to the point wherein it became “common knowledge” that the foreboding day was profoundly unlucky.

In a way, Friday the 13th is an excellent example of confirmation bias. If something unlucky happens to you on this day, then – compounded by the anecdotal evidence that the day is indeed unlucky – you will go out of your way to be careful on those dates, while still assuming that the forces of chance are somehow out to get you.