Logical Gal and post hoc ergo propter hoc

29 Jan

When our boys were little they liked to flaunt a Latin phrase their Dad taught them over dinner:

“Post Hoc, Ergo Propter Hoc” 

It’s Latin for ‘after this, therefore because of this’.  Let’s take two events that occur, one after the other. What happens is that people ASSUME that in this parallel occurrence,the first event actually causes the second.  But this isn’t necessarily true.  The two events just HAPPEN to be linked.  It’s pure coincidence.

The above bread example shows how silly this thinking can be.  But I encounter it frequently as a teacher.

Me:  John, why don’t you ever study for your history tests?

John:  Because I studied once and got an F!  Therefore, if I study, I do worse!

John probably thinks that his response, however faulty it is, suffices to get annoying  teachers or parents off his back! But armed with reason, we can counter and call him on his error. Unfortunately, teens are not the only ones susceptible to this fallacy.

The idea of jinxes or superstitions are another area where people indulge in ‘stinkin thinkin.’   Maybe one time, on a lark, you performed a series of actions or incanted a string of words and good happened to ensue!  So, your thinking goes, it was what you said or did that caused the happy outcome!  But anyone with a pea pod of sense would be able to show how unscientific that is!

My husband is an engineer and has taught me another version of this fallacy: Correlation does not imply Causation.  Just because two things are related does not automatically mean that one causes the other.

Yet, don’t we live, ever trying to prove causation!  If I have heartburn, I’ll think back to what I ate, to see if there is a connection.  I’ll narrow down the foods, through trial and error, trying to pinpoint a true cause of the heartburn.

So maybe this fallacy, whether you call it by its fancy Latin name of Post Hoc ,Ergo Propter Hoc or its more plain-jane name of False Cause, happens because people don’t pursue their experiments long enough to actually establish cause and effect!  To guard against coming to a faulty conclusion, carefully study not just the events that happen and the order they occur.  Look for other factors, other possible reasons for something to occur. Don’t assume what you might prefer to conclude!  Be rigorous.

I’ll leave you with one thought. Although we have dealt with mostly frivolous examples, this line of thinking can be compelling when parents are faced with the unknowns of a disease like autism.  Some people have linked childhood vaccinations TO a diagnosis of autism.  This may or not be true.  But the consequences of going unvaccinated can be a matter of life and death!

Question: What are some examples from your life where you actually have succumbed to the Post Hoc Fallacy?

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