Can you identify the major fallacy common to the following statements?
1. Drinkers are more likely than non-drinkers to get lung cancer, therefore drinking causes lung cancer.
2. The lacrosse team gets better grades than the baseball team, therefore playing lacrosse makes you smarter than playing baseball.
3. Our blog views were down in April. We also changed the color of our blog header in April. This means that changing the color of the blog header led to fewer views in April.
4. Yesterday, I walked under a ladder with an open umbrella indoors while breaking a mirror in front of a black cat. And I forgot to knock on wood with my rabbit’s foot. That must be why I’m having such a bad day today. It’s bad luck.
Post hoc ergo propter hoc (translation: “after this therefore because of this”) is a common logical fallacy because we have some trouble distinguishing correlation and causation. We tend to think if one event comes before another, the first must have caused the second. Or if two events happen in unison, we tend to think there is a connection of causality between the two. This isn’t always the case.
In sentence 4, the theoretical possibility exists that various objects or situations can bring about bad luck; however, that is something that has never been demonstrated to be true. Also, the idea of ‘luck’ may be a hard thing to measure. We may not share the same views about what constitutes luck. In other words, the ‘bad luck’ here is to some degree a matter of interpretation, and there is no reason to assume that a non-natural relationship exists between the events mentioned in the sentence and the ‘bad luck’ of this particular day.
In sentence 1, there actually is a strong correlation between drinking alcohol and lung cancer, but it isn’t because alcohol causes lung cancer. It’s the cigarettes smoked while drinking. We have to check for possible ‘third causes.’
Sentences 2 and 3 demonstrate this ‘third cause’ idea very clearly: the lacrosse team may study more than the baseball team so good grades aren’t necessarily directly related to playing the sport. And changing header colors on a blog may not be the actual reason for views to drop off—it may be that the blog is just boring.
Key points: This fallacy does not refer to the future or whether something has occurred. Both things must have happened. Frequently, there is a third cause (an unmentioned event) for the things observed, so look for it. And finally, a little bit of skepticism is called for when examining claims of causation. Being extremely skeptical can call into question any relationship and its cause, but a ‘reasonable doubt’ is in order.