It’s logical fallacy hour again, here in Skeptictionary corner. These tend to be a little lighter and less research-intensive, and I don’t want to wait another month before being able to post something else, so I’m scaling down a bit from the recent mammoth on homeopathy. [Spoiler from the future: I ended up rambling on for over a thousand words on this anyway. But at least it only took me a week this time.]
Therefore, A causes B
So, what’s the deal with the latin up there in the main title? Damn Romans, you’d think they invented being wrong, the way they get to name everything about it.
Post hoc ergo propter hoc translates to “After it, therefore because of it”. It refers to the idea that, because two things were seen to happen in sequence, the thing that happened first must have (or probably) caused the thing that happened next. It’s fallacious reasoning because, as is commonly pointed out, correlation does not imply causation.
I had a hole in my sock when I attended an event or competition featuring my local team of sportsmen or athletes, and they won! It must be my lucky sock!
I let a spiritual homeopathic healer acupuncturate my feet with his natural quantum reflexopractic needles, and just two weeks later my minor cold had been completely cured!
I did a traditional tribal rain-dance for three days straight, and sure enough the heavens finally opened, and the gods gave us water for our crops!
The Family Guy gag (the last of the list) highlights how ridiculous this kind of reasoning can be, but it’s often more subtle and pervasive than that, and the first three quotes are exactly the kind of ideas that do genuinely persuade people that they’ve discovered some secret magic which gives them power over the universe. Throw in a good dose of confirmation bias, and it’s easy to become convinced that your choice of tattered footwear can affect a soccer game several miles away, and to write off all the times it hasn’t worked as minor, irrelevant aberrations.
We like finding patterns to things, and it’s a big advantage in nature to be able to connect related concepts and predict the future. If I know that tigers tend to make rustling noises in bushes, then when I hear a rustling bush I can run away before I see the tiger, without having to have his presence confirmed by seeing his teeth where my arm used to be. This sort of low-level prognostication comes in handy for a burgeoning species.
But it’s an instinct that can lead us astray, in our enthusiasm to build up a neat, logical picture of how things in the world are ordered, because sometimes things aren’t very neat or logical. When two things appear to be occuring in tandem, there might be other things going on more complicated than simply “therefore A causes B”.
B causes A
I know that post hoc implies a temporal sequence, so the thing that happens later can’t really go back in time and cause the thing that already happened first. So maybe I’m really talking about the more general cum hoc fallacy (“with it”, rather than “after it”), but whatever. Sometimes you might just have the causitive effect backwards.
Hospitals must be unbelievably dangerous places to go. Have you seen how many sick and dying people are in there?
You know, it’s pretty suspicious how the police always seem to turn up after a crime’s been committed. Returning to the scene to admire their handiwork, perhaps?
That kind of thing, though there are probably some less silly examples I could have thought of if I’d got more sleep last night.
Some third thing C causes both A and B
In this case too, the correlation is real. You will likely find that instances of these two things you’re looking at tend to go together. But it’s not simply that one causes the other – there’s actually something deeper going on beneath both of them.
I’ve noticed that people with grubby teeth seem to get lung cancer more often than the rest of us. Does the cancer spread to the teeth? Or is the yellow stuff they get on their teeth giving them cancer?
Everyone keeps getting presents when they put a tree inside their living room. I guess their hospitality is being rewarded by a generous wood-nymph.
One way this kind of fallacy can lead you astray is if you start trying to change one thing by manipulating the other, when actually they’re both just side-effects of some deeper principle, and don’t affect each other at all. (Say, putting up a Christmas tree in June and waiting expectantly for the gifts to accumulate underneath it.) A fascinating example of this can be seen in cargo cults, of which more at some indeterminate future date, maybe.
Blind luck and dumb animals
There’s a lot of stuff going on in the world at any particular moment. There are many variables which go up and down over time. Sometimes, some of these will line up for a bit, with no underlying significance whatsoever, purely by coincidence. Pastafarianism makes good use of this by attributing global warming to a lack of pirates (or possibly vice versa), but a lot of more common magical thinking falls into this category as well.
Get barely over a thousand people flipping coins, and it’s more likely than not that one of them will get ten heads in a row on their first try. If you look hard enough, there’s probably something unique about that one person, to which you can attribute this “luck”.
And if 20,000 people regularly turn up to sporting events and take note of exactly what they’re wearing each time, some of them are going to find pretty convincing patterns between their chosen attire and the performance of their team. (And, again, some of them will see entirely unconvincing patterns but remember the hits and forget the misses.)
I’m sure we’re all mature enough here to be beyond such utter bullshit, but the whole “lucky socks” thing, as I’m going to categorise it, pisses me off enough to be worth focusing on for at least another paragraph. I know you want to feel important, and it’s tempting to leap to what seem like justified conclusions, but can you really not get over that instinct and just grow the fuck up? If you honestly believe that any ritual you compulsively go through actually has any effect on the “luck” of something as unconnected and multivariate as a Cup final, then you are literally too retarded to be allowed to handle crayons without supervision. And by “literally” I of course mean “figuratively”, but you’re almost certainly too moronic to know the difference.
Why would your own magic talisman cancel out every other equally lucky object owned by every single other person watching this sports game, let alone the relative skills and efforts of all the actual players? By what inanely trivial divine law would the outcomes of such events revolve solely around a single banal and irrelevant action by, of all people, you? If circumstances like sports results and the weather are going after you personally, why do they completely ignore everybody else’s schedule but your own?
You are seeing patterns where none exist.
Pigeons believe this kind of thing, or rather they learn to perpetuate arbitrary behaviour patterns in their efforts to achieve an unrelated goal. They’re not animals known for a natural talent for critical analysis, but they’re just about smart enough to realise that, if they were tapping a pattern with their left foot when the food turned up last time, it might be worth tapping it out again to see if it still works. But we’re really supposed to be more intelligent than fucking pigeons, and we really ought to have grown up beyond the point where we think there are mischievous leprechauns pushing footballs around mid-flight based on whether some guy watching the match is wearing the right underwear, or whatever the fuck the logic’s supposed to be.
Stop ranting, this is supposed to be one of your serious and informative bits
Sorry. Where was I? Oh, I think I was about done.
I’ve said before that humans have a crappy natural grasp of things like probability, and maybe that deserves a whole post of its own. But we have things like science, and statistics, which mean we don’t need to rely on our appalling instincts in determining the truth. We can do some tests and look more closely at alleged relationships like this, and if there’s really something there, we can find out more about it. But if we’re not doing any of that, then we’re wandering blindly in a world of wrong ideas, with no way of knowing how misguided are the concepts we’re snatching at.