In this video I briefly describe the history of intelligence testing including the work of Sir Francis Galton, Alfred Binet, Theodore Simon, William Stern, Lewis Terman, and David Wechsler. I also discuss the concepts of mental age, ratio IQ, deviation IQ, and the ability of childhood IQ to predict future outcomes.

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Video Transcript

Hi, I’m Michael Corayer and this is Psych Exam Review. In this video I’m going to provide a brief history of intelligence testing and we’ll start with Sir Francis Galton. Here’s a picture of Galton here and Galton is considered to be the founder of the field of psychometrics. This is because he was a bit obsessed with measuring people and also because he invented the statistical technique for calculating correlation.

Now Galton was interested in intelligence and he believed that it was inherited. In 1869 he published a book called “Hereditary Genius” in which he followed family histories and genealogies of eminent minds in an attempt to find this association between intelligence and inheritance. Now he was also influenced by the ideas of his half-cousin Charles Darwin and he believed that if intelligence is inherited then that means it must offer some sort of survival advantage.

And then Galton figured if intelligence provides some sort of survival advantage, then maybe that means it can also be seen in certain body measurements. Maybe there’s something about the survival advantage that shows itself in other ways. And so Galton measured all sorts of features of people; measured their reaction time, their sensory acuity, their muscular power, their body proportions, and even their head size and attempted to connect these to intelligence. But unfortunately he wasn’t able to find any particularly strong correlations between these measurements and intelligence.

Now at the end of the 1800s, the French education system was undergoing reform and this meant that there was now free mandatory schooling for all children. The problem was that children had widely different educational backgrounds. Some had attended school regularly, some had attended school only sporadically, and some hadn’t attended school at all. And all of these children needed to be brought together into a single educational system. And so what there was a desire for was a system that would allow students to be assessed in order to place them appropriately for the maximum benefit. So if there were students who had not attended school at all they would be quite far behind and they might need remedial classes to catch up. Whereas students who had been attending school already might already be doing well and being in the same class with these other students might hold them back from their full potential. So students couldn’t simply be sorted by their age because their educational backgrounds were so different. Now this led to the creation of an assessment for these students and this was created by Alfred Binet and Theodore Simon. And here’s a picture of Alfred Binet and here’s a picture of Theodore Simon.

And what Binet and Simon created was the Binet-Simon test and this was to help sort students appropriately in order to put them in classes where they would benefit the most. Now an important concept that came out of this Binet-Simon test was mental age. And so the idea of mental age was that it provided the average performance for students of a particular age. So when Binet and Simon gave this multifaceted examination to students of all different ages, they were able to calculate the mental age. So they said here’s the average performance for a particular age. In other words, this is the performance that most six-year-olds showed on this assessment. And that could be referred to as a mental age of six. And this meant that students could now be judged by their mental age not just their chronological age. They could be said to, okay this child is five years old, but he’s performing at a mental age of seven. Or maybe this child is eight years old but he’s only performing at a mental age of six, right? He’s performing as well as most six-year-olds.

So this concept of mental age was then adopted by a German researcher named William Stern and here’s a picture of William Stern. And Stern used this to create an intelligence quotient or IQ and so the idea of an intelligence quotient was it’s a comparison between mental age and chronological age. So if we look at somebody’s mental age and we divide it by their chronological age this will give us some fraction, which is then multiplied by 100 and this gives us what’s called a ratio IQ score.

So for instance if a child is five years old, chronological age of five, and performs at a mental age of most five-year-olds, it has a mental age of five. Then five over five would be one, times 100 would mean that the child would have an IQ of 100. And this is why the average IQ is a hundred because that’s when mental age and chronological age match. So if you’re seven years old and you perform like most seven year-olds, then you have an IQ of 100. Now if you’re five years old but you perform at a mental age of six, then you’d have an IQ of 120. Alright, so if your mental age is higher than your chronological age that gives you an IQ over a hundred and if your mental age is lower than your chronological age then you end up with an IQ below one-hundred.

Now this works pretty well for assessing children. The problem is it doesn’t work very well once we get out of childhood because the idea of mental age doesn’t work so well beyond a certain point. So what’s the difference in mental age between an 18 year old and a 19 year old? Or a 29 year old and a 28 year old? Or a 58 year old? You know at that point it’s very difficult to compare things by mental age. And so we have this problem where chronological age keeps increasing every year, you know, as you get older your chronological age is getting, the denominator of this fraction, is getting larger and larger. But the concept of mental age isn’t working very well to compare and so this is why ratio IQ is not calculated much anymore. Instead researchers today use another technique called deviation IQ.

So the difference between a ratio IQ and a deviation IQ is, what a deviation IQ does is it just compares your score to the average score for your age range. So the average score for your group, and then multiplies that by 100 and that’s how you get this deviation IQ, And again we can see the average IQ will still be 100. If you score just as well as the average for your group then you’ll have an IQ of 100. If you score above the average of your group then your IQ will be above 100 and if you score below the average for your group then your IQ score will be below 100. So this allows us to make comparisons. Let’s say you’re 25 years old, then we can compare your score just to the performance of other people who are around that same age.

Now this brings an important point about IQ which is that IQs not about comparison across age ranges in terms of specific performance. So what I mean by that is, if you take a child who’s five years old and they score 130 on an IQ test that doesn’t really tell us about what they can actually do. So I might compare this five-year-old with a 130 IQ to a nineteen year old with an IQ of 90. Now the 19 year old can solve a lot of problems that the five year-old can’t yet solve, right? So the 19 year-old is doing algebra 2 equations and probably knows a lot more words and definitions of words than the five year old but that’s not what the IQ score is trying to tell us. The IQ score is about the potential ability of the five-year-old. It’s saying that we predict that this child can learn a lot and by the time he’s 19 we can assume he’s going to be performing better than this current 19 year old with the IQ score of 90. But we’re not comparing them directly and that’s why we have this deviation IQ score that only compares your performance to other people in your group.

Ok, so let’s get back to our history of assessment and this brings us to Lewis Terman and here’s a picture of Terman here. Terman was at Stanford University and he was influenced by the work of Binet and Simon and what he did was he adapted the Binet-Simon test and this became the Stanford-Binet Intelligence Scale. And he first did this in 1916 and the test is actually still in use today although it’s currently in its fifth edition. And the original tests combined work from the Binet-Simon test with William Stern’s idea of ratio IQ, but the current version of the test now uses deviation IQ.

Now Terman is most famous for a longitudinal study that he began in 1921 and what he did was he gave this Stanford-Binet Intelligence Scale to thousands and thousands of children. He assessed about 168,000 children and of these he picked out 1,500 of the top-performing children, and these were children who had an IQ score of 135 or higher on this Stanford-Binet Intelligence Scale. And then he followed these children throughout their lives and he wanted to see whether early childhood IQ could predict future performance in life. He found that these children in the high IQ group published more academic papers, they registered more patents, they wrote more novels, they were more likely to have higher levels of education, and higher job status and higher income when they were adults compared to children who didn’t perform this highly on the IQ test.

Now this showed that early IQ really could predict certain future outcomes. Now it wasn’t a guarantee; it wasn’t the case that all of these children achieved high levels of success but it showed that there was a relationship. And it also showed that some of the myths that existed at the time, such as the idea that high IQ children were more likely to become depressed, or they were more likely to burnout and sort of fail later in life, Terman showed that these were indeed myths and this didn’t seem to be the case.
That early childhood IQ was associated with higher levels of education and income and job status in the future and this study actually continued for many decades even after Terman passed away.

Right, now I said that the Stanford-Binet Intelligence Scale is still in use today but it’s not the most popularly used intelligence scale. And this brings us to our final researcher that we’ll look at and this is David Wechsler. Here’s a picture of Wechsler here and Wechsler created an assessment in 1939 that was first called the Wechsler-Bellevue Intelligence Scale but it’s now changed its name and it’s in its fourth edition. And we have what’s called the Wechsler Adult Intelligence Scale.

And there’s actually two other versions of the Wechsler test. So there’s the Wechsler Adult Intelligence Scale, there’s also the WISC this is the Wechsler Intelligence Scale for Children, and for very young children we have the Wechsler Primary and Preschool Scale of Intelligence. Now all of these intelligence assessments involve multiple parts they test. They assess a number of different things and an important point is that you might have this idea that intelligence tests are like pencil paper multiple choice types of questions. That’s generally not the case and if you took one of these Wechsler assessments then it would be in person with a trained psychometrician and most of the responses would be verbal.

There’s actually not a lot of writing on these types of intelligence tests. So you’d be asked things about general knowledge, you’d be asked to explain phrases or word meanings, you’d be tested for your memories of things like a span of digits that you’d be asked to remember, and you’ll be asked to apply rules to particular situations. And all of this would then be assessed by the psychometrician and that will be used to determine your intelligence score.

Ok, so I hope this gave you a little bit of a better understanding of different types of intelligence tests and where they come from and in the next video we’ll talk about the results of intelligence tests and we’ll talk about variation between individuals and what that variation might mean. I hope you found this helpful, if so, please like the video and subscribe to the channel for more. Thanks for watching!