Reasoning Skills and Determining Causes Help (page 2)

Updated on Sep 29, 2011

Looking for the Common Denominator

Sometimes, the cause can be determined not by looking for what's different, but by looking for what's the same—that is, something that each incident has in common. Take the following scenario, for example:

    Jason has been having trouble sleeping a few nights a week. On the nights when he can't sleep, he notices that the neighbor's dog is always barking and howling. Jason concludes that his trouble sleeping is due to the dog.

Jason has used a logical approach to determine the cause of his insomnia. He's looking for a pattern—something that is consistent with the nights he can't sleep. Because he hears the dog barking and howling on those nights, it could be that the dog is preventing him from getting his sleep. The dog is the common denominator for all of these occasions.

Just as it is important to be careful not to overlook other possible differences, however, it's important to remember to look for other possible common denominators. Before Jason concludes that his sleeplessness is because of the dog barking, he should carefully consider whether there might be anything else in common on those nights that he can't sleep.

So let's complicate the situation just a bit by adding more evidence from which to draw your conclusion.

Jason has been having trouble sleeping a few nights a week. On the nights when he can't sleep, he notices that the neighbor's dog is always barking. He also realizes that the sleepless nights are always nights that he hasn't talked to his girlfriend. Moreover, those are nights that he skipped going to the gym because he worked late. What's causing Jason to have trouble sleeping?

  1. the dog barking
  2. not talking to his girlfriend
  3. not exercising
  4. none of the above

Can you answer this question with confidence? Probably not. That's because each of these answers is a legitimate possibility. Each situation occurs on the nights Jason can't sleep. Just like the coffee may not have been the only thing different in the previous scenario, here, the dog isn't the only common denominator. There are many possibilities. If you're to confidently say which of these is the cause, you need to pinpoint just one event in common with all the bad nights.

If Jason knew that the dog barked every night—even on those nights when he is able to sleep—then the barking dog could be eliminated as an option. Similarly, if Jason skips the gym on other occasions when he can sleep, then choice c could be eliminated. But until more evidence is given and the other possibilities can be eliminated, none of the choices can be chosen over the others.

Post Hoc, Ergo Propter Hoc

Nina, who'd always dressed rather plainly, decided it was time to jazz up her wardrobe. She went shopping and bought a closet full of new, brightly colored clothing. Two weeks later, she was promoted at work. "Wow," she told her friend, "I had no idea that what I wore to work could make such a difference. Just changing my wardrobe finally got me that promotion I'd been waiting for!"

Nina deserves congratulations, but not for her reasoning. What's wrong with her logic here?

Nina has committed the post hoc, ergo propter hoc inductive reasoning fallacy. Post hoc, ergo propter hoc literally means after this, therefore because of this. Nina has assumed that because her promotion came after she changed her wardrobe, her promotion was caused by her change in wardrobe. Maybe, just maybe, her appearance did have something to do with it. But in all likelihood, there were several other causes for her promotion. She'd probably been doing good work for months or years, for one thing, and the position to which she had been promoted may not have been vacant before. There may be several other reasons as well.

Of course, cause and effect is a chronological structure—the cause must come before the effect—but remember that you need to consider other possible causes. Just because A comes before B doesn't mean there's a logical connection between the two events.

    Here's another example of post hoc:
After the Citizens First Bill was passed, crime in this area skyrocketed. Funny how the bill that was supposed to reduce crime actually increased it!

Notice how this argument assumes that because the Citizens First Bill came first and the rise in crime came second, one caused the other. But proving that there's a link between the two events would not be easy, especially since an increased crime rate could be caused by many different factors. In fact, a figure as complicated as crime rate is probably caused by a multitude of factors. What else can you think of that might have caused the increase in crime?

    Other possible causes:

You may have listed other possible causes like the following:

  • An increase in unemployment
  • A recession
  • A change in population in the area
  • A reduction in the police force

In fact, because human society is so complex, most social issues have multiple causes. In all likelihood, the increase in crime was caused by a combination of these, and possibly other, factors. But the Citizens First Bill, unless it specifically cut jobs and reduced the police force, is not to blame. It may have come first, but it's not necessarily the cause.


Cause and effect can be illustrated in many of the science experiments you have done in class. Think about being in the lab. What happened when you combined certain elements? They were the cause. What was the effect?

This seems like a reasonable argument, not a post hoc error. Part of the logic comes from the fact that bike riding works the calf muscles a great deal and could easily cause cramps.

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