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Since Thanksgiving break is here, and I had some free time, I decided to write a blog post on something I noticed on the Tumblr psychology tag the other day. It seems most persons on there believe that just because two things seem related, this automatically means that one causes the other.
Imagine the following hypothetical scenario:
While writing a paper on crime rate in Metropolis, a reporter, almost by chance, notices that as the city’s overall consumption of ice cream increases, so too does the murder rate increase in the city. A positive correlation between ice cream and murder rate is uncovered. In other words, as ice cream sells increase so does the murder rate:
As you can see, the more gallons of ice cream consumed by the city, the more homicides that occur. However, what does this mean? Does it mean that consuming more ice cream makes people in Metropolis more violent? Or, on the other hand, does it mean that as murder rates increase, citizens eat more ice cream to ease their nervousness and calm their stress?
The fact of the matter is we can’t be too sure. Maybe eating more ice cream increase murder, or vice-versa. Or maybe these two variables (ice cream consumption and murder rate) are influenced by a third variable.
Notice the bottom of the chart: As the months get closer to summer, both ice cream consumption and murder rates increase. So maybe a third variable, heat, causes these results. An increase in heat causes people to go out and buy ice cream to cool themselves down. In a similar way, the increase in heat makes certain persons more aggressive and prompts them to commit murder. (Keep in mind this is only a hypothetical example; don’t go out telling your friends that more people get killed during hotter weather [now that I think about it, that sounds like an interesting topic for a future blog post].)
As you can see, just because two variables are correlated to one another does not mean that one directly causes the other. In fact, it can be the case that both influence one another. An example of this might be the relationship between watching violent movies and teen violence. Some may argue that watching violent movies cause teen violence. However, others may point out that teens who are already violent seek out violent movies, so, in fact, being violent leads to watching violent movies. Or, it can be the case that a third variable influences both. For example, a child may act violent because his parents act violent; this same child watches violent movies because his parents watch violent movies.
There are sophisticated methods researchers can use to figure out whether one thing truly causes another. However, these methods are not always employed in correlational studies. Keep this in mind the next time someone tells you that A directly causes B for the simple reason that when A rises, B also rises.