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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.

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References:
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

In popular culture, the term IQ is everywhere.

People who boast about their IQ are losers.

When people say IQ, what they mean is intelligence, an objective, rigorous measurement of intellectual ability.

Well, in this video, I wanna find out where IQ came from, what does it actually measure, what can it predict about your life, and I guess what is my IQ?

I have never taken an official IQ test before.

Honestly, I don't think I'm terribly smart.

I've always kind of considered my IQ to be maybe just a little above average.

There are a lot of IQ tests online, but I am very skeptical about their accuracy.

Still, I figured some of them may be good practice for the real thing.

Tomorrow I'm going to do an IQ test for real.

Before I do that, I want to try to improve my score.

And so I'm going to try to do a whole bunch of practice tests.

I think this test is trainable, but tomorrow we're going to see whether that's true or not.

The idea of intelligence testing goes back hundreds of years, but the first concrete breakthrough occurred in 1904.

English psychologist Charles Spearman was studying students' grades in different subjects, and he wondered how their performance in 1 subject like English would relate to their performance in another like math.

1 option would be that the better a student did in math the worse they would do in English, maybe because they spent more time on their math work and so had less time to devote to English.

So performance in different subjects would be negatively correlated.

Another option was that performance in 1 subject would be completely unrelated to performance in another.

After all, different subjects require different skill sets, so maybe marks would be totally uncorrelated.

The third option was that the better a student did in math, the better they would do in English.

In other words, their marks would be positively correlated.

A correlation coefficient can vary anywhere from negative 1 to positive 1.

A correlation coefficient of negative 1 indicates a perfect negative correlation, meaning an increase in 1 variable corresponds to a precise, predictable decrease in the other variable.

Similarly, a correlation of positive 1 indicates a perfect positive correlation.

A correlation of 0 indicates no relationship between the 2 variables.

And any value between 0 and 1 indicates a positive correlation, but the data has some random spread.

The square of the correlation coefficient tells you the amount of variation in 1 variable that can be explained by variation in the other variable.

For example, if the correlation coefficient is 0.5, then 25% of the variation in 1 variable can be explained by the other.

When Spearman analyzed his data, he found a clear positive correlation.

Students who did better in math also tended to do better in English, and the correlation coefficient was 0.64.

But math and English weren't the only subjects the student studied.

And when Spearman looked at the correlations between all of these subjects, he found the same pattern.

Students who did well in 1 subject tended to do well in them all.

Well, Spearman proposed that each person has some level of general intelligence, what he called the G factor.

This construct was meant to capture how quickly students could learn new material, recognize patterns, and think critically, regardless of the subject matter.

Which explains why students' scores across subjects are correlated.

Those with high G score well on all subjects, and those with low G score poorly on all subjects.

Spearman published his conclusions in a paper titled General Intelligence Objectively Determined and Measured.

So on top of the G factor, Spearman proposed subject specific factors or S factors.

A student's performance in math, for example, would depend on their general intelligence plus their subject-specific factor for math.

Subject-specific factors could increase or decrease performance on that particular subject.

Spearman believed that specific factors could be trained, but general intelligence was fixed.

So he wanted to find a way to reliably measure general intelligence.

At around the same time in France, Alfred Binet was tasked with figuring out which kids needed more help in school.

Together with Theodore Simone, he developed the Binet-Simone test.

Students were asked to name what's missing in the drawing, define abstract terms, and repeat back sentences.

And there was also this question asking which face is prettiest.

Their performance was benchmarked against other students of different ages in order to assign them a mental age.

For example, if a student performed about as well as the average eight-year-old, their mental age would be 8.

This mental age was then divided by their actual age and multiplied by 100 to arrive at a so-called intelligence quotient.

So the Binet-Simone test was the world's first IQ test.

It was translated by Goddard into English and brought to the US.

At Stanford, Lewis Terman standardized it using a large American sample.

And with some modifications, it became the Stanford-Binet test.

And for decades, it was the most widely used test in the United States.

They all had the same goal of measuring the G-factor.

The way they did this was by assessing many different mental abilities, including memory, verbal, spatial, and numerical skills.

Each 1 of these areas might have a subject specific shift but by averaging them all together, the idea was the subject specific effects would cancel out leaving a decent approximation of G.

Of course, there would always be some error, but that's why psychologists designed IQ tests with upwards of 7-10 sections with distinct tasks to try to minimize subject-specific distortions.

All the different IQ tests differed in the number of questions and their difficulty.

So to standardize the scoring system, each test was given to a large sample of the population.

Raw scores were normalized, usually so the mean was 100 and the standard deviation was 15.

And that's how it's still done to this day.

This is known as IQ, and it's meant to be a measure of an individual's g-factor in comparison to the rest of the population.

The way it's scaled, 68% of people have an IQ between 85 and 115.

Only around 2% score over 130 or under 70.

11 lions, 4 cats and 7 crows have a total of...

As I was studying for my IQ test, I practiced all the different types of questions that appear on modern tests.

1 section will almost certainly be on vocabulary.

They give you 1 word, like sanguine, and you have to pick which of the multiple choice options is most similar in meaning.

Is it gloomy, asinine, recalcitrant, optimistic, or reflective?

They might also ask you to pick a word with the opposite meaning.

So what is the opposite of perspicacious?

Is it canny, obsequious, dull, fanciful, or sagacious?

Another section tests your ability to spot patterns with numbers.

So pick the number that best completes the pattern.

Originally I was looking for complicated patterns, but as I familiarized myself with the online tests I discovered the patterns were usually pretty simple.

A good technique is to find the difference between adjacent terms.

So in this case the first 2 terms are separated by 2, the next by 3, and then 4.

So the logical next term should be 5 more than 12, so 17.

Sometimes the numbers grow rapidly, like in the sequence 3, 15, 60, 180.

In cases like this, I look at the ratio of 1 number to the 1 before it.

In this case, the second number is 5 times the first.

The next number is 4 times bigger and the next 1 is 3 times larger.

So the answer should be 2 times the fourth term, which is 360, answer B.

1 of the best known types of IQ test questions are Raven's progressive matrices.

These involve a 3 by 3 grid with symbols in each of the cells.

And you have to select the ninth cell, which follows the pattern.

I found that the bulk of these puzzles obey 1 of only a few different logical rules.

1 is translational motion, so the symbols move from 1 cell to the next in a predictable fashion.

The second is rotational motion, 1 or several objects rotate from 1 cell to the next.

The third is missing symbols, where in each row or column each symbol appears once.

So to figure out which symbols appear in the final cell, you just have to spot which ones are missing.

And the fourth is addition, where the first cell plus the second cell equals the third cell.

In this case, lines that overlap cancel out, but a line plus nothing equals a line.

In most modern IQ tests, all the questions are completed under time pressure.

You may have only around 10 to 30 seconds per question.

Okay, this morning I'm taking an official IQ test, and I gotta say that I'm pretty nervous.

I always want to do well on tests, you know, it's something I pride myself on, but at the same time, you know, who knows how this is going to go.

I'm not allowed to take you in there because obviously people don't want the questions getting out and they don't even want video of what it looks like in there.

You know, they're very strict about these things.

So I'm going to go in, do the test, I'll come out, and I'll let you know how it went.

What's remarkable about IQ tests is that an hour or 2 of questions on vocabulary, numbers, and arbitrary shapes can predict a surprising amount about your life.

For 1 thing, the higher your IQ, the larger your brain is likely to be.

A large meta-analysis from 2005 estimated a correlation of 0.33 between IQ and brain size.

In 2007, Scottish psychiatrist Ian Deary measured the IQs of 13, 000 11-year-olds.

And 5 years later, when these students completed national school examinations, Deary compared their exam marks to their IQs.

Their performance on an IQ test when they were 11 correlated with their performance 5 years later on the GCSEs.

About 0.8, that's an extremely high correlation.

It means about two-thirds of the variation in national school examination scores could be predicted by IQ tests taken 5 years prior.

Now the correlation coefficient of this study is at the high end of the 0.2 to 0.8 range found in similar studies, but research supports the claim that IQ is a good predictor of school success.

It also predicts how much schooling a person will complete.

Maybe this shouldn't be so surprising, since some school tests are essentially IQ tests.

It's been argued that tests like the SAT, ACT, and the GRE are basically IQ tests.

They correlate with standard IQ tests at around 0.8.

Now on my SATs, I got a score of 1330, which corresponds to an IQ of around 130.

So it'll be interesting to see if my official IQ score matches that, or if I was able to increase my score by familiarizing myself with IQ style questions.

But IQ also has predictive power outside of school.

1 of the most robust findings is that IQ can predict job success.

Particularly in technical or high complexity jobs.

You measure productivity in ways that economists use about, you know, the output generated.

The correlations typically range from 0.2 to 0.6.

And the effect is most notable for more complex jobs, which makes sense.

The highest effect is for military training.

In fact, the US military will not accept anyone with an IQ under 80.

They also limit to 20% the number of recruits with IQs between 81 and 92.

During the Vietnam War, in order to increase the pool of applicants, they relaxed this last requirement.

But what they found was that those below the threshold were 1.5 to 3 times as likely to fail recruit training, and they required between 3 to 9 times as much remedial training.

Taken together, this added so much strain that the military ran more efficiently without the extra recruits.

In total, 5, 478 people recruited under this initiative died at a fatality rate 3 times higher than ordinary recruits.

So the military reinstated their requirements and today anyone with an IQ less than 80, that is about 30 million Americans, would be ineligible to join the military.

Even outside the military, IQ seems to play a role in how long you live.

In a Scottish study, scientists uncovered IQ tests from kids when they were 11 years old.

Now, 65 years later, they checked to see who from the sample was still alive at age 76.

And they found that on average for every 15 point increase on the IQ test, you would be 27% more likely to still be alive at age 76.

A large meta-analysis confirms that people with higher IQs have a lower risk of dying during the time frame investigated in each study.

The last major thing that IQ seems to predict is income.

This study shows a clear tendency for income to increase with IQ, and it found a correlation coefficient of 0.3.

In fact, the top 3 earners in this study all had IQs below 100.

A large meta-analysis of 31 studies found the correlation between IQ and income to be 0.21.

It means that only 4.4% of the variance in income is explained by IQ.

Maybe 1 of the reasons why we don't see as high a correlation for income is just because economically, intelligence is not necessarily that highly rewarded in that maybe there are jobs, you know, like just doing a real estate type scheme.

Like maybe that doesn't require a huge amount of intelligence.

Simultaneously, you have all these very highly intelligent people who maybe become college professors, but that doesn't necessarily pay very well.

lot of people who have very high intelligence scores don't have the same interest in accumulating money.

The relationship with net worth is even weaker.

It hardly seems to correlate with IQ, even though people with higher IQs are supposedly more intelligent and on average, they make more money each year.

But this apparently doesn't translate into saving or accumulating more wealth overall.

But if IQ correlates with school achievement, job performance, income, and longevity, why don't we hear more about it?

I think it's because IQ has a dark history.

When Henry Goddard brought Binet's test to America, its use and interpretation shifted dramatically.

In France, Binet believed intelligence could be improved through education.

He designed his tests so that struggling students could be given more help to catch up.

But in the US, the modified test was given to adults to rank them by intelligence.

And researchers like Spearman believed that G was unchangeable, that whatever general intelligence you were born with, you would have for the rest of your life.

And many thought G was inherited, passed down from parents to children.

These days we would say it has a genetic basis.

There is some evidence to support these assertions.

IQ appears fairly consistent over one's lifetime.

So they had tests done when people were 11 years old.

They found all those tests in a filing cabinet and followed those people up and gave them the same test when they were 90 years old, you know, much, much later.

Their scores, 80 years apart, were correlated at around 0.5 to 0.6.

There's also evidence for a genetic basis to IQ.

You find, for instance, that if you get 2 identical twins and give them an IQ test, they have a very strong correlation.

It's actually about the same as giving the same person the test, you know, a few weeks apart.

Henry Goddard used the claims that intelligence was inherited and unchangeable to put IQ at the center of the American eugenics movement.

Eugenicists wanted to prevent those with undesirable traits from having kids.

In many states, laws were passed to enable forced sterilization of people who failed to meet a certain threshold on an IQ test.

The constitutionality of these laws was upheld by the Supreme Court in 1927.

Even words that we now use as insults, moron, idiot, imbecile, were used as scientific terms.

In his judgment, Justice Oliver Wendell Holmes wrote, it is better for all the world if instead of waiting to execute degenerate offspring for crime, or to let them starve for their imbecility, society can prevent those who are manifestly unfit from continuing their kind.

3 generations of imbeciles are enough." In total, over 60, 000 people were forcibly sterilized as a result of these laws.

In fact, they served as a model for Nazi Germany.

Hitler himself claimed to be inspired by American eugenicists.

These have been used for horrific things in the past.

At the Nuremberg trials after the war, some Nazis quoted from the American Supreme Court decision.

Given this awful history, I think it's understandable that many people completely disregard IQ today.

On the science of intelligence, there are a number of things those early researchers got wrong.

1 is that IQ is not entirely determined by genetics.

Can you quantify the effects of genetics versus environment?

When you look at twin studies, on average across the whole lifespan, it's about 50-50 to heritability and environment.

You simply can't, for ethical reasons, estimate it in humans, with a reasonable degree of certainty or accuracy.

Given my reading of that literature, it's a pretty broad range, probably somewhere between 40% and

And since education can improve IQ, it is not completely fixed over a lifetime.

Plus, intelligence might not be a single construct as initially imagined.

These days, scientists recognize 2 forms of intelligence, fluid and crystallized.

Fluid intelligence is your ability to learn, process information, and solve novel problems, whereas crystallized intelligence involves the knowledge you've accumulated over your lifetime.

Both types of intelligence increase throughout childhood, but fluid Intelligence peaks in early adulthood and then declines, whereas crystallized intelligence remains more stable.

But IQ has been further misused to promote the idea of racial differences in intelligence.

There is, for example, an observed gap between the average IQ of black and white Americans.

Articles have also been published on the IQs of different nations around the world.

Many of these nations are purported to have average IQs below 70.

That's the cutoff for intellectual disability.

The conclusion that some draw is that there are genetic differences between races or nations in intelligence, but I think that's a gross misrepresentation of the data.

The problem, I'd argue, is that IQ tests don't necessarily measure what you think they're measuring.

And the proof is that there's a representative sample of white Americans whose average IQ is 70.

Just ordinary Americans who lived around a hundred years ago.

Researcher James Flynn studied the average results of IQ tests over the past century.

And every so often the tests get updated and renormalized to keep their average at a hundred.

Now what Flynn noticed was that each time they got renormalized, the scores had to be shifted down a bit more, by about 2 or 3 IQ points per decade.

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What Does IQ Actually Measure?