Common Thinking Traps – Correlation and Causation

by Stephen Mills on January 25, 2010


Correlation Does Not Prove Causation

Note from Stephen: There are a number of hot button issues mentioned in this article.  In general, except where noted, I’m not indicating a preference for any explanation, nor am I intending to start a debate on any of these issues.  These are merely examples of alternatives that can be used to illustrate the main point of the article.

People who have been beaten as children are more likely to beat their own children.  Therefore being beaten as a child makes the child more likely to grow up and become a child beater himself.

This thinking trap is an incredibly widespread problem that shows up daily in media reports.  A is correlated with B so therefore A must have caused B.

If there is a correlation between A and B, there are multiple possibilities.

  • A may cause B
  • B may cause A
  • A third factor C may cause both A and B
  • A and B may influence each other in some kind of reinforcing relationship
  • It might be a coincidence that A and B are correlated
  • Some combination of the above

Some of them can be ruled out in certain cases.  Beating your child cannot have been the cause of your having been beaten as a child, so the second possibility (B causes A) can be ruled out in that example.

Similar to the child beating example, children of smokers are more likely to become smokers.  Therefore smoking causes your children to become smokers themselves.  That kind of narrative sounds plausible and we hear it all the time.  The implication is that children are learning these behaviors from their parents.  While that is certainly plausible, another just as plausible explanation is that children are genetically inheriting these tendencies from their parents.  The genetic explanation for both violence and smoking have a lot of supporting evidence from adoption studies.

Socioeconomic class is correlated with all kinds of things, therefore socioeconomic class must be the cause of all those things.  A classic example is crime.  Poor people commit more crimes, therefore the cause of some crime is poverty.  Maybe, but then maybe something else is causing both crime and poverty.  Some argue for exactly that and it seems that your preferred explanation depends upon your politics.

Children that are read to by their parents do better in school.  Therefore, reading to your child helps him or her do better in school.  Maybe a third factor such as a genetic trait for academic talent causes you both to do better in school and to be more likely to read to your children.  It seems likely that academically talented parents and educated parents are more likely to read to their children.  They also gave their children some of their academic genes and that could be the explanation.

Children in music programs do better in math, therefore you should enroll your child in music programs so they will do better in math.  Maybe, but then maybe the same trait makes one both good at music (and thus more likely to participate) and good at math.  There is a correlation between being in a high school orchestra and being Asian.  If you want to become an Asian, should you join the orchestra?

There is a correlation between ice cream sales and drowning incidents.  Neither is likely the cause of the other.  Instead a third factor is likely increasing both – summer.  Marijuana is the gateway drug to more dangerous drugs.  We are told this all the time and it is proven by the fact that those who smoke marijuana in their youth are more likely to take more dangerous drugs later on.  This plausible sounding explanation ignores the just as plausible explanation that someone with a personality or genetic tendency to take drugs is more likely to both smoke marijuana as a youth and take other drugs as they get older.  What caused them to be a drug user when they are older might be the same thing causing them to smoke marijuana when they are younger.

Here is one of my favorite examples because it got a lot of attention and seemed so scientifically acceptable.  A correlation was discovered between young children sleeping with a light on in their bedroom and development of near-sightedness.  It was implied that leaving a light on in your child’s room leads to near-sightedness.  It turns out the likely cause of this effect is that near-sighted parents transmit their near-sighted genes to their children and those same half-blind parents are more likely to leave the light on in their child’s room, possibly so the parents themselves can see better.

A properly controlled experiment can tease out likely causes, but much of what you hear in the media is not the result of carefully controlled experiments.  It is the result of people, often with an agenda, looking for correlations.  We don’t conduct controlled experiments on child beating and youth drug use.

They key lesson here is to not just automatically assume that because the media reports that people who do A are more likely to experience B, means that A is causing B.  It pays to be skeptical of media reports, books, and gurus when they imply correlation indicates a casual link.  This is especially true if it sounds like something that was just dug out of data or surveys and not the subject of proper experiments.  If you care about the issue, you should look deeper into the studies.

What do you think?  Leave a comment and join the conversation.


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{ 22 comments… read them below or add one }

Eduard @ Ideas With A Kick January 25, 2010 at 2:41 pm


This phenomenon is the one single reason I loved my experimental psychology class in college. Most people get away with all sort of conclusions because they wrongly use correlations to sustain causation. Especially those who quote some research to prove something. I rarely let them get away with that.

All the best,

.-= Eduard @ Ideas With A Kick´s last blog ..Effective personal development starts here =-.


Stephen Mills January 26, 2010 at 6:47 pm

Hello Eduard. That’s great. Our brains look for patterns so we want to find explanations and correlations are the ideal for that. I often find myself wanting to fall into the trap. On the other hand, in a well designed experiment a high correlation can indicate a significant effect.


Positively Present January 25, 2010 at 3:28 pm

This is really interesting stuff, Stephen! Thanks so much for writing about it. You made the topic really relatable and interesting!
.-= Positively Present´s last blog ..make the 3-to-1 ratio of positivity work for you =-.


Stephen Mills January 26, 2010 at 7:03 pm

Hi Dani, thanks for all your support 🙂


Nimit Kashyap January 25, 2010 at 5:27 pm

some good points you mentioned
.-= Nimit Kashyap´s last blog ..WordPress Redirection Plugin To Manage 301 Redirects =-.


Stephen Mills January 26, 2010 at 7:05 pm

Hello Nimit. Welcome and thanks for the comment.


patti January 26, 2010 at 9:14 am

this is a scientific approach that my right-brain struggles with… maybe that’s because both my parents are creative people and I grew up valuing art and literature more than all things science and math? well, no; nothing could be further from the truth. thanks for this perspective – I look forward to keeping up with the conversation.
.-= patti´s last blog ..Bloggers Unite for Haiti =-.


Stephen Mills January 26, 2010 at 7:12 pm

Hi Patti! My left brain struggles with the right brain stuff 🙂


Amit Sodha - The Power Of Choice January 26, 2010 at 1:40 pm

I very rarely listen to what I hear in the media. I don’t personally drink but one week you’ll hear, drinking alcohol is good for so and so, the next week you’ll hear, alcohol is bad for so and so.

When it’s all put into perspective it’s always better to come up with your own understanding of the truth through research.

There is so much biased information out there it’s very important to find out for yourself. Great post Stephen
.-= Amit Sodha – The Power Of Choice´s last blog ..Honouring People And Cultures With Language Skills =-.


Stephen Mills January 26, 2010 at 7:15 pm

Hi Amit, you hit it on the head with this:

“When it’s all put into perspective it’s always better to come up with your own understanding of the truth through research.”

The Internet is fantastic for research but it’s not to good at filtering quality. You have to do that yourself.


Tess The Bold Life January 29, 2010 at 4:28 pm

I agree with every word! So many people are acting like the sky is falling and it’s not. The only thing falling are the old systems. Only if they fail will the new ones be put into place and this book is asking us to be part of the new. Count me in!
.-= Tess The Bold Life´s last blog ..Green Living – Made Simple =-.


solidmastery January 31, 2010 at 10:20 am

Finally. After all these years of pretending to know the difference from correlation and causation, I get it.
Thanks TheRatRaceTrap.
I guess I learn best by a series of examples.

Proceeding, I will have to examine all 6 angles before I espouse a theory, form a conjecture, or argue a point.

Were we supposed to have learned this in school?


Richard | February 2, 2010 at 4:48 am

Arghh we had a psychology teacher in school who used to say this EVERY DAY! Great memories. So true as well. I used to thin that Man Utd lost every time I went out to watch them. It change with the City and Arsenal games though 🙂
.-= Richard |´s last blog ..Lessons From a Month of Meditation =-.


Vin - NaturalBias February 4, 2010 at 8:39 am

Hey Stephen,

Great topic! It’s especially relevant to epidemiological research. Correlations give us clues and sometimes even help us identify a cause, but they don’t prove the cause.
.-= Vin – NaturalBias´s last blog ..A1 Beta Casein: The Devil in Your Milk =-.


Holly April 28, 2013 at 6:40 pm

I’m so sick of hearing that kids who play music do better in school. The band director at my kids’ school just sent a flyer around that said something like in an experiment children who were given piano lessons and computer math puzzles did better on proportional math problems than children who were not given those. Okay, if they’re so sure that piano improves math, then why add the computer math puzzles? I really enjoyed what you wrote about this and will share it with the band director after my kids graduate. I was never in the band at school, but I was one of the top three math students who got to go to the math competition.


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