12 Days of Scepticism: Day 5- Post Hoc Ergo Propter Hoc

Post hoc ergo propter hoc; this is one of my favourite logical fallacies, not just because it sounds cool but because it’s so commonly used and easy to spot. It translates as ‘after this therefore because of this’ and the essence of it is that a conclusion has been reached based solely on the order of events as opposed to any actual evidence. Here’s the pattern:

A happened then B happened

Therefore B happened because of A

An example: someone has a bad back or a gammy knee or whatever and goes out seeking the help of their friendly neighbourhood quack; let’s say a chiropractor or acupuncturist – it doesn’t matter which as they’re both crap. They receive their placebo and, would you believe it, within a matter of days they start to feel better. As their recovery came after their treatment they fallaciously assign efficacy to said treatment without any evidence that it was anything other than regression to the mean.

As well as regression to the mean this fallacy can also be inspired by confusing correlation with causation along with a big dollop of confirmation bias. Along with the Natural Fallacy it’s pretty much the cornerstone of complimentary and alternative medicine so watch out for it whenever you hear anyone rattling on about that. It can crop up almost anywhere though: rituals in sports, lucky charms, ESP, gambling, astrology. You name it, people can confuse it.


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