|Source: Fallacy-a-Day Podcast|
Like most fallacies, it's not too hard to see why people engage in it. We humans are pattern seeking animals, which is something that saved our lives when we discovered the pattern that the big hairy things with sharp teeth regarded us as lunch rather than good friends. Unfortunately, we sometimes see patterns that aren't really patterns at all. Take a look at the clouds sometime. See that rabbit? Yeah, you ain't seeing a rabbit. Your brain is trying to make sense of random data is all.
The "post hoc" argument consists of attributing something to an event that preceded it. For instance, I once had a headache and then I had a bowl of Cap'n Crunch cereal. Shortly after I finished it, my headache was gone. Therefore, eating Cap'n Crunch cures headaches, right? Umm...no.
Making these conclusions isn't always a fallacy. When your cat scratches your arm and you bleed, it's perfectly logical to conclude that a cat scratch can make you bleed. The difference between that and the Cap'n Crunch example is that you can explain the mechanism of how a cat's claws cuts through your skin. There is no equivalent to the Capn' Crunch scenario. Plus, there are several other factors that can go into a headache going away - one of which may have been that I was simply dehydrated and the milk took care of the problem. I could have had a glass of water and achieved the same result. (And sometimes headaches just go away on their own and it's best to just say that you don't know why it did.)
When considering whether you're engaging in this fallacy or not, it's best to step back and consider all the other factors that could be going on with the situation in question. In other words, step outside of yourself and realize that your personal experience may not represent the bigger picture.
For instance, where I see this fallacy used the most often is with the hysteria over vaccinations. People point to their children showing signs of autism shortly after receiving their vaccinations. Also, you'll hear stories of kids getting really sick shortly after their vaccines. Then you'll get numerous stories of these vaccines supposedly being the cause of the various ailments. While it is true that vaccines aren't completely free of side-effects, it's not logical to attribute every negative thing that happens after one to the vaccination.
In the case of autism supposedly being the cause, the age that kids receive their vaccines (two years old) is also the age where one would expect to first see signs of autism. Another important thing to consider is what happens when you flip this idea around? Think of how many millions of kids get vaccinated and DON'T have autism. Why not say that vaccines prevent autism, but there's a low failure rate and some of them become autistic anyway? After all, there are kids who haven't been vaccinated who have autism.
Of course, what I just wrote is a logical fallacy, and nobody is even trying to make the claim that vaccines prevent autism. (Most likely because nobody gets hysterical when a negative reaction DOESN'T happen.) However, it's the same argument, only it's backed up with better numbers than the "vaccines cause autism" side. In other words, it's illogical, but it's even less illogical than those who would get you in a panic over vaccines.
Regarding the kids who get sick after their vaccines, again, there are potential side-effects (although it's safe to say that none of them are as bad as the potential side effects of whooping cough, measles, etc.) but the thing is this, KIDS GET SICK. And they get sick for a number of reasons. We don't always know why, but grasping at the easiest answer doesn't mean you've found THE answer. What's wrong with just saying, "I don't know what happened?"
And again, step outside yourself. What if you not only have a personal experience where your kid suffered a negative reaction but what if you also knew somebody else who did? What if you ten people? A hundred? A thousand? Well, what's that compared to the entire world population that's been vaccinated? Nothing, really. Now, if we were getting reports all over the world where pretty much every kid was suffering from serious negative side-effects right after being vaccinated, then we'd have something. But as of right now, you're still at a level with my Captain Crunch example, and coming up with more fallacies won't suddenly turn your fallacy into a truth.
My son had some respiratory problems before he turned one. He doesn't seem to be showing any signs of them now. What was the cause? What was the solution? I don't know, and pretending to know would be disingenuous, even though I could probably come up with all sorts of reasons.
It must have been all of those Disney princess movies I let him watch. (Which also explains why he loves dance class so much.)