What Does IQ Actually Measure?

What Does IQ Actually Measure?

IQ is supposed to measure intelligence, but does it? Head to https://brilliant.org/veritasium to start your free 30-day trial, and the first 200 people get 20% off an annual premium subscription.

If you’re looking for a molecular modeling kit, try Snatoms – a kit I invented where the atoms snap together magnetically – https://ve42.co/SnatomsV

A huge thank you to Emeritus Professor Cecil R. Reynolds and Dr. Stuart J. Ritchie for their expertise and time.

Also a massive thank you to Prof. Steven Piantadosi and Prof. Alan S. Kaufman for helping us understand this complicated topic. As well as to Jay Zagrosky from Boston University’s Questrom School of Business for providing data from his study.

Kaufman, A. S. (2009). IQ testing 101. Springer Publishing Company.

Reynolds, C. R., & Livingston, R. A. (2021). Mastering modern psychological testing. Springer International Publishing.

Ritchie, S. (2015). Intelligence: All that matters. John Murray.

Spearman, C. (1961). ” General Intelligence” Objectively Determined and Measured. – https://ve42.co/Spearman1904

Binet, A., & Simon, T. (1907). Le développement de l’intelligence chez les enfants. L’Année psychologique, 14(1), 1-94.. – https://ve42.co/Binet1907

Intelligence Quotient, Wikipedia – https://ve42.co/IQWiki

Radiolab Presents: G. – https://ve42.co/RadioLabG

McDaniel, M. A. (2005). Big-brained people are smarter: A meta-analysis of the relationship between in vivo brain volume and intelligence. Intelligence, 33(4), 337-346. – https://ve42.co/McDaniel2005

Deary, I. J., Strand, S., Smith, P., & Fernandes, C. (2007). Intelligence and educational achievement. Intelligence, 35(1), 13-21. – https://ve42.co/Deary2007

Lozano-Blasco, R., Quílez-Robres, A., Usán, P., Salavera, C., & Casanovas-López, R. (2022). Types of Intelligence and Academic Performance: A Systematic Review and Meta-Analysis. Journal of Intelligence, 10(4), 123. – https://ve42.co/Blasco2022

Kuncel, N. R., & Hezlett, S. A. (2010). Fact and fiction in cognitive ability testing for admissions and hiring decisions. Current Directions in Psychological Science, 19(6), 339-345. – https://ve42.co/Kuncel2010

Laurence, J. H., & Ramsberger, P. F. (1991). Low-aptitude men in the military: Who profits, who pays?. Praeger Publishers. – https://ve42.co/Laurence1991

Gregory, H. (2015). McNamara’s Folly: The Use of Low-IQ Troops in the Vietnam War; Plus the Induction of Unfit Men, Criminals, and Misfits. Infinity Publishing.

Gottfredson, L. S., & Deary, I. J. (2004). Intelligence predicts health and longevity, but why?. Current Directions in Psychological Science, 13(1), 1-4. – https://ve42.co/Gottfredson2004

Sanchez-Izquierdo, M., Fernandez-Ballesteros, R., Valeriano-Lorenzo, E. L., & Botella, J. (2023). Intelligence and life expectancy in late adulthood: A meta-analysis. Intelligence, 98, 101738. – https://ve42.co/Izquierdo2023

Zagorsky, J. L. (2007). Do you have to be smart to be rich? The impact of IQ on wealth, income and financial distress. Intelligence, 35(5), 489-501. – https://ve42.co/Zagorsky2007

Strenze, T. (2007). Intelligence and socioeconomic success: A meta-analytic review of longitudinal research. Intelligence, 35(5), 401-426. – https://ve42.co/Strenze2007

Deary, I. J., Pattie, A., & Starr, J. M. (2013). The stability of intelligence from age 11 to age 90 years: the Lothian birth cohort of 1921. Psychological science, 24(12), 2361-2368. – https://ve42.co/Deary2013

Flynn, J. R. (1987). Massive IQ gains in 14 nations: What IQ tests really measure. Psychological bulletin, 101(2), 171. – https://ve42.co/Flynn1987

Why our IQ levels are higher than our grandparents’ | James Flynn, TED via YouTube – https://www.youtube.com/watch?v=9vpqilhW9uI

Duckworth, A. L., Quinn, P. D., Lynam, D. R., Loeber, R., & Stouthamer-Loeber, M. (2011). Role of test motivation in intelligence testing. Proceedings of the National Academy of Sciences, 108(19), 7716-7720. – https://ve42.co/Duckworth2011

Kulik, J. A., Bangert-Drowns, R. L., & Kulik, C. L. C. (1984). Effectiveness of coaching for aptitude tests. Psychological Bulletin, 95(2), 179. – https://ve42.co/Kulik1984

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Written by Derek Muller, Casper Mebius, & Petr Lebedev
Edited by Trenton Oliver
Filmed by Derek Muller, Han Evans, & Raquel Nuno
Animation by Fabio Albertelli & Ivy Tello
Additional video/photos supplied by Getty Images & Pond5
Music from Epidemic Sound
Produced by Derek Muller, Casper Mebius, & Han Evans

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39 Responses

  1. Mahir Nagersheth says:

    I just took an IQ test and I am SO happy…

    Thank God it came back negative!

  2. Ithecastic says:

    I’m reminded of Goodhart’s law: “When a measure becomes a target, it ceases to be a good measure.”

  3. John Chessant says:

    30:12 I feel like you should’ve mentioned Lewis Terman’s “genetic studies of genius” here. This was a longitudinal study where kids in the Bay Area (since Terman worked at Stanford) were given IQ tests, and Terman would keep tabs on the highest scorers and compare their eventual life outcomes to the general population. Contrary to his expectation, the vast majority of his “Termites” had mundane adulthoods and fared no better than a random sample of people with similar socioeconomic backgrounds. And the real kicker: two eventual Nobel Prize winners, physicists William Shockley and Luis Walter Alvarez, grew up in the Bay Area and were tested, but did not score high enough to be included in the study.

    • IMightBeBiased says:

      Some funny stuff happens in a lot of those Stanford studies. When they went back to the marshmallow test kids 40 years later, that study that supposedly showed that patience as a kid helps you later in life, the results showed no statistical difference between the patient kids and the impatient ones in any meaningful category.

    • Crimson Spork says:

      Probably because we have population wide studies showing high IQ predicts exactly what you would think it predicts like salary.

    • One Horse Too Many says:

      @IMightBeBiased It’s possible that over half of soft-science studies are unreproducible. This is doubly true if they involved children.

    • aceman0000099 says:

      He didn’t really mention individual studies, because he looked at meta-analysis studies. Lewis Terman’s results might have shown no correlation, but 20 other very similar studies (as you can see in the video) gave significantly different correlations. You really need a sample size of 8 billion to get any accuracy, even then there will be flaws in the testing strategy.

    • One Horse Too Many says:

      Children develop at different rates, and just a few months equivalent development can make a huge difference in their scores. Even if kids are the exact same age, to the day, the differences in how fast they develop makes the results extremely unreliable. Wait until they’re at least 25 if you want meaningful IQ test results.

  4. xxwookey says:

    The practice part really matters. When I did this as a kid I got 9 or 10 points higher after practicing some tests to get used to the question types. Which in itself illustrates that intelligence testing is hard.

    • Brian Mcdaniels says:

      Uh., All this proves is that the testing is based on knowledge and not intelligence (aka the capability to learn anything)

  5. Arnaud Morin says:

    It would be fantastic to have a follow-up video on the distinction between intelligence and rationality, for example by tapping into the work of Keith Stanovich. And Derek, that could be an opportunity to go back to some of the early Veritasium videos: Stanovich developed a rationality test measuring the RQ (rationality quotient), with questions that remind of questions you used to ask people in the street, like “would you take that bet?” and generally measure people’s ability to avoid logical fallacies or to display competent data literacy (which you discussed in videos like the one on the regression to the mean or the one on Kahneman’s system 1 and system 2). The rationality test is also more interesting than the IQ test: it is possible to have a high intelligence and still perform poorly on rationality questions. And when you see how Dan Kahan showed how intelligence is often used to protect biases, measuring rationality is a better predictor of the type of citizen someone will be and of the behaviour they will have regarding important issues like politics or climate change.

    • DerDean_HD says:

      Wow that sounds very interesting, I really hope Derek reads your comment.

    • Offensive Architecture says:

      It is so random, I just learned about RQ today reading an academic paper. Yeah I am curious about this concept of RQ and how it differs from IQ. It would be a really interesting video topic.

    • Aaron Davis says:

      And the thing with both IQ and RQ is, _personality_ makes both of them it’s bitch. High IQ people can wind up failing badly, for example going to prison, when they are narcissists, prone to emotional decision making and rage. While IQ (and presumably RQ) are both correlated with success, the best childhood predictor of later success in adulthood is not IQ but deferment of gratification. The famous _marshmellow test_ shows astonishing predictive capacity for success.
      Of course, low IQ people can have personality flaws; they may even be more likely to than high IQ people. But having a high IQ won’t count for much if your life is in chaos, and you’re probably better off having an average IQ and a dependable personality than having a high IQ and being a sociopath, for example. The perfect case study that exemplifies this is Ted Kaczynski.

    • Marcelo Coutinho says:

      Found a smart comment, nice!

    • theBear89451 says:

      I’m going to predict R=0.8 on this one.

  6. Sarah Marshall says:

    When I did my ASD and ADHD profiling, I did an IQ test. It was actually super helpful for understanding how my brain works as a neurodivergent person. My verbal IQ is 132 and my abstract reasoning is in the 99th percentile. But my working memory score was 83, and my digit span score was in the 3rd percentile lol

    It helped me understand why I’m so smart and so dumb at the same time ? Like I can understand complex topics quickly and with very little context, but when I worked at subway while I was at uni, I had to ask what they wanted on their sub 5 times bc I couldn’t remember 😅

    • Chris Banana says:

      I just got done with my ASD and ADHD testing, at age 37, and yeah out of the 16 hours of testing in total, there was a lot of IQ type testing mixed in there 😀 I actually loved the pattern ones, i kept wanting to do more 😀 also the spatial testing was really neat!

      edit: oh the “copy this crazy nutty drawing!”, then 10mins later, “remember that drawing? yeah draw it again….” then the next day!, again….
      And on the 2nd day lots of random questions about the drawing, like “which one of these “patterns” was in the abstract drawing that you did yesterday”, or a “was this in the drawing from yesterday?”
      I really want to know my score on that one. I felt good on my drawings, even on the 2nd day, but the “was this really in there?” really got to me, like i was getting false memories

  7. Alex Smogy says:

    Cecil: you’re smarter than 98.8% of the population
    Derek: wow😒
    Cecil: hopefully you’re not disappointed
    Derek: *visibility disappointed*😕

  8. Joshua Erkman says:

    The best explanation of correlation coefficient I’ve heard definitely going on my statistics playlist.

  9. Supernovah ' M says:

    I always found the hardest part about having to take these tests was being in a classroom with closed doors, not being able to move or make noise, dealing with the noise of other people fidgeting or clicking/whatever and the social construct of appearing to find something difficult or easy in front of other students. With the much harder questions I felt observed and analysed and would attempt to jot down answers before I had actually arrived at some conclusion. I always performed fairly well but not as well as I could have due to the constraints forced upon me by the examination conditions. As a young child, most people predicted I would be wildly successful in academia and life in general. And although now I am finally getting there – I struggled for many years with the sorts of things most people don’t ever encounter. Social isolation, abuse, living in an orphanage, and some other facets – lead me to a life of complications. I finally published in an academic journal for my first time (an IEEE one) ten years after my peers of the same age. I also feel more capable than I perceive them to be; working faster and developing more robust understandings more readily than my peers. Anyway, hope you enjoyed my little anecdote.

  10. Frey Finley says:

    I had an IQ test taken after I burnt-out in highschool. Where with most people the results on the different types of tests line up more or less, my scores had differences between them of at least 30 points- The average was 130 and you’d expect someone with a score like that to do well, but because the scores are so different it’s hard to make up for the parts I lack. It’s like my brain is constantly full of errors.
    I now know, after getting a diagnosis, that having a wonky score like that is a relatively common thing with autistics (I also have ADHD and Dyslexia, which can also be a cause of such weird results).
    IQ may be high, but I can’t work without burning-out within a year. I can’t live on my own or take care of myself.
    Don’t consider that one IQ score as holy.

    • Aaron Cottle says:

      I did not know there were so many of us wow this comment section is full of ND people explaining how not very useful the number is to them.

    • Aeggy says:

      @Aaron Cottle What is ND?

    • David Willson says:


    • 3. 1415926535 says:

      Hey there, fellow ND. I can definitely relate to that. except my IQ scores were always below average, even though pretty much everyone around always tells me I’m intelligent. I’ve been called genius by some (which I thoroughly disagree). Anyway, much like you, I’m the kind who can easily grasp difficult mathematical ideas, but cannot cook my own meal without setting the neighborhood on fire.
      Also, burnout is like a ghost that keeps haunting me.

    • dzarko55 says:

      Hey, literally the exact same situation here (minus dyslexia). 131 average, lowest category was 98 (!). The others were stable just above 131. And I can’t manage my life – at all. So I relate.

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