January 5^th, 2009

Ever heard of the Post Hoc Fallacy? Or, more properly, Post Hoc Ergo Propter Hoc ("After this, therefore because of this.")? It goes like this:

1. First something happens.
2. Next, something else happens.
3. Therefore, that thing in Step 1 must have caused that thing in Step 2. Q.E.D.

And people like to play this game with themselves all the time: "I prayed to God for a sign, and the next week there was an Earthquake in Indonesia. There really is a God!" Or, "A study found that teenagers who smoke get lower grades; therefore, smoking lowers grades." (Or, sometimes, "Quit smoking to raise your GPA.")

Taken to its extreme (as if those two examples aren't already extreme enough), "I sneezed and a building in Lichtenstein fell over, so I made the building collapse!"

This one part of critical thinking seems to be too much for most of us to fathom (avoiding the post hoc fallacy). So often, one hears reports that link something in our lifestyle to some sort of malady or a life expectancy. Examples are everywhere:

• People who drink one or two drinks a day have lower instances of heart disease. (OK so far.) Therefore, if you don't already, you should starting having a couple of drinks every day in order to reduce your risk of heart disease. (Faulty logic.)
  There are so many reasons why those two are linked: Perhaps whatever makes people live longer also makes them want to drink. Or perhaps they would live even longer if they abstained. The facts are not sufficient to support the conclusion.

• I have a cold, and discover that if I eat 9 oranges and drink a litre of chicken soup I get better within 2 or 3 days. Therefore, 9 oranges and a litre of soup constitutes an effective cold remedy. (Faulty logic.)
  Or just waiting 2 or 3 days is equally effective ... or drinking a litre of any fluids does the trick, or ... etc. etc. etc. We just don't know.

• Or how about a real life example: Johanes Fibiger discovered roundworms in the stomachs of rats that had stomach cancer. He concluded that those worms were the cause of stomach cancer.
  Nonsense: It could have been a coincidence, it could have been that whatever food(s) caused cancer had these worms in it, it might have been that cancer weakened the rats' defence against these parasites ... heck, it could have been almost anything at all. But to conclude the worms caused the cancer is ridiculous. Unfortunately, this logical fallacy was not caught by the Nobel prize committee, who graciously rewarded Fibiger's faulty reasoning skills with $1,000,000 and the Nobel Prize for Medicine. (I bet that's a dark day in the annals of the Nobel history that members of the organization would rather we all forgot.)

There are many more examples, but those three serve to show the kinds of faulty post hoc fallacies in which we can get ourselves trapped.

The problem here is that there really is something called Cause and Effect. I push a wineglass off the counter, it falls and smashes on the kitchen floor. Now there's a real conclusion: The glass broke because I pushed it off the counter. Because there really are causal statements we can make, we have to evaluate every set of events carefully and on an individual basis to determine whether or not there really is a valid cause-and-effect conclusion that can be drawn.

It requires critical thinking, and sometimes the reasoning is unclear or too complex for us to be sure of its veracity. And we don't seem to be learning it in schools ... it makes it difficult to apply critical thinking if we haven't had at least a little bit of practice at it.

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