Ask Science uncovers the truth (and lies) of the correlation/causation fallacy. Just because something seems to cause something else, does not necessarily mean it does.
So if this is such a well-known fallacy, why does it show up so often? The place where this fallacy shows up the most often is in media headlines, which unfortunately is where most people get their science information and news. Imagine you’re looking to buy a magazine. Which headline best grabs your attention:
“One study on a limited population shows that when people do X, Y happens a certain percentage of the time.”
“Link found between doing X and Y happening!”
“New research shows that X causes Y.”
If you ever read a scientific paper, you’ll find that almost all scientists make statements like that first one. However by the time this research hits the popular media, it’s often transformed to look a lot more like that last one.
See also: How the Media Sensationalizes Science
Why We Care About Correlations
So if correlations are such rubbish, why do scientists spend so much time telling us about them? The thing is, that while no scientist believes that correlation necessarily means causation, to a scientist, a correlation between two things can be like a signpost that helps guide them to the truth.
Imagine that you’re trying to figure out your boyfriend’s favorite kind of ice cream so you can buy him some for his birthday. You don’t want to give away the surprise by coming right out and asking him. However, you’ve noticed that whenever he goes out for ice cream with his friends, he always comes back with a chocolate stain on his shirt. In other words, chocolate shirt stains are correlated with going out for ice cream.
Now you might want to jump to a conclusion here and say, “The chocolate stain is caused by him eating chocolate ice cream!” but that would be succumbing to the correlation fallacy and you’re smarter than that. Maybe one of his friends always manages to spill chocolate ice cream on your boyfriend’s shirt; maybe they’re involved in some kind of male bonding that requires them to throw ice cream at each other; it might not even be plain chocolate, maybe it’s rocky road, or fudge ripple. The possibilities are endless.
So you decide to use your stain observations to come up with a hypothesis that can be tested. You hypothesize that your boyfriend likes plain chocolate. To test your hypothesis, you buy some chocolate ice cream for yourself and offer him a bite. He turns his nose up at it and says that plain chocolate is too boring.
While you haven’t discovered the truth yet, you can use this new evidence to refine your hypothesis. You look for more correlations, noticing how much he seems to like marshmallows. A few days later you offer him a bite of rocky road. His eyes light up and a broad smile stretches across his face as he takes half of your ice cream in a single bite. Success!