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You’ve probably heard of ‘IQ,’ but do you really know what it means? This lesson covers the history and theory behind the famous idea of an intelligence quotient (including the inventor of IQ, Alfred Binet), and will help you learn how to understand and interpret IQ scores.

What is IQ?

Imagine you’re a ten-year-old child who lives on a farm in France during the turn of the century, around the year 1904. You’re having trouble in school, learning how to spell all those French words and how to read books like Candide. But why are you struggling? Is it because you are low in intelligence? Or is it simply because you are from a poor family who doesn’t have the same resources as a rich family? Maybe you are very intelligent, but your problems in school are due to pragmatic issues such as a lack of books in your house.A man named Alfred Binet lived in France during this period and noticed that many children in school were struggling, but he didn’t think it was due to lack of intelligence. So Binet made a very important distinction between two different things that might lead to poor school performance.

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He said that some children will do badly because they simply aren’t intelligent, so there’s not much that can be done to directly help them improve their grades. However, he said that other children have plenty of learning ability and potential to do well, but they struggle simply because of life circumstances. He said that these children, who have a high learning ability, should be distinguished from children who do badly in school because of low intelligence.

Alfred Binet made a distinction between high potential and low intelligence children
chronological age. What’s your chronological age? Right now I’m 36, so that’s my chronological age.

But Binet also said that we need to know your mental age, which is your cognitive abilities relative to what others can do of different ages. In other words, what do you think an average five year old can do? Can they read? Can they do math? What about the average ten year old? Or how about someone who is 40 years old? Your mental age is what you are capable of doing, scaled to what the average people of different ages can do. In other words, if you’re capable of doing what an average 25-year old can do, then your mental age is 25. If you can do what an average 10-year old can do, but not anything more advanced than that, then your mental age is only 10.How did Binet figure out what an average kid of each different age could do? He simply did research using a bunch of kids at each age to directly measure their cognitive abilities. Instead of measuring academic knowledge like, ‘Who wrote the play Romeo and Juliet?’, he measured ability to solve problems and think creatively, such as noticing patterns or remembering images. After testing lots of kids of all different ages, he could then give you, personally, the same test and compare your scores to what kids of each different age could do.

If your scores matched what the 10-year olds were doing, then we know your mental age is 10 years old.So now that we have these two scores – chronological age and mental age – we want to compare them. If you are right on track, then your two scores will perfectly match. If you are advanced, or very intelligent, according to Binet, then your mental age will be bigger than your chronological age. Say, if you are 20 years old right now, but you can do what an average 30-year old can do, then you’re 10 years ahead of schedule, which Binet would say makes you very intelligent. The opposite could also be true; maybe you’re 20 years old, but can only do what the average 10-year old can do. Binet would say that you are cognitively delayed and therefore not intelligent based on IQ.

To get a bit more technical, Binet created an equation so that we could all have a quick, simple number that indicates where we stand in comparing these two scores. The equation looks like this: Mental Age / Chronological Age x 100 = IQ. We can make a ratio of the two ages, with mental age on top, and the only reason we multiply by 100 is to get a nice, round number instead of a fraction.

Let’s go through a couple of examples of the simple math here.If your mental age and chronological age are a perfect match, you might get an equation like this: 20 mental years old / 20 actual years old x 100 = 100. You can see that an IQ score of 100 will always mean that a person is exactly where the average person would be, so an IQ of 100 will always mean average intelligence for IQ. If a person is mentally advanced, they might get a score like this: 30 mental years old / 20 actual years old x 100 = 150. An IQ score of 150 would be very impressive! Just as an example, the rumor is that Albert Einstein had an IQ of 160! Any IQ higher than 100 means that the person is a little bit more intelligent than average. Again, the opposite can also be true.

Let’s say we have a person who is a little bit developmentally delayed; he or she might have an equation like this: 10 mental years old / 20 actual years old x 100 = 50. An IQ score less than 100 will always mean that a person’s mental abilities are not as good as the average person of his or her age group.

Interpreting IQ Scores

You already know that an IQ score of 100 means average intelligence, that anything over 100 means advanced intelligence and that anything under 100 means delayed intelligence.

Binet did these calculations back in France in the early 1900s. Note that Binet thought of children who scored lower than 100 as not necessarily low on intelligence but as simply delayed, or behind schedule. The French word for ‘delay’ is ‘retard.

‘ The original use of the word ‘retarded’ simply indicated this mental delay in academic abilities. Unfortunately, the word now has been misused and misunderstood and is considered very rude in modern times.Once you know someone’s IQ score, you can now get a basic idea of whether they are on track, delayed or ahead of schedule. But there’s more to understand about how the scores work. For example, just how intelligent is someone with an IQ score of 120 compared to 140? How rare is an IQ score of 160, like the one Einstein was supposed to have?Almost all modern IQ tests have standardized their scores, so that results can be interpreted very specifically along a standard bell curve. In a different lesson within Educational Psychology, you can learn about the theory and interpretation of bell curves in general, so we’ll skip that for this lesson.

For now, all you need to know is that standardized IQ tests are always set up so that the average score will fall right at 100 (just like Binet wanted) and that the standard deviation for scores is always 15 points. That means that the majority of people will get an IQ score that’s within 15 points above or below the average of 100. In other words, most people will get a score between 85 and 115.Most IQ scores are set up so that their scores will fall on a bell curve that looks just like the one here on the screen. You can see that the standard deviation of 15 points groups most people right in the middle; this range between 85 and 115 makes up 68% of everyone who takes the test.

34% of all people get a score between 85 and 100, and another 34% of people get a score between 100 and 115. So that takes up about two thirds of the population. You can also see on this graph that if you go out to two standard deviation scores from the middle (so that’s 30 points on each side), you’ll have scores that were either below 70 or above 130.

Again, you can see that these very low or very high scores are extremely rare. Only about 2% of the world gets a score of 70 or less, and only about 2% of the world gets a score of 130 or higher. So Einstein was in a very small group of super smart people!

Most IQ scores fall between 85 and 115
IQ Score Distribution

IQ in School Today

While IQ scores are not quite as popular in modern schools as they were a few years ago, some schools still use IQ tests to identify students who have special needs.

Students who seem to have developmental challenges might be given an IQ test to see just how delayed they are so that they can be given specialized attention in school. This will help them succeed in the best way for them. On the other end, talented and gifted students might be given IQ tests to see just how advanced they are so that they can be placed in advanced courses or given special opportunities in after school programs.Have you ever taken an IQ test? Do you know where your score falls in the bell curve?

Lesson Summary

In summary, IQ stands for intelligence quotient, which is your cognitive abilities, relative to what others can do of different ages.

The concept of IQ was invented in the year 1904 by Alfred Binet. The equation used to calculate a person’s IQ score is Mental Age / Chronological Age x 100. On most modern IQ tests, the average score will be 100 and the standard deviation of scores will be 15.Using a normal bell curve, you can see how common or rare any given IQ score is, with about two-thirds of scores falling somewhere between 85 and 115.

Many modern school systems still use IQ tests to identify students who have special needs, due to either very high or very low scores.

Lesson Objectives

After watching this lesson, you should be able to:

  • Identify Alfred Binet and explain how he developed the IQ equation
  • Differentiate chronological age and mental age
  • Understand the significance of a person’s IQ and how the scores generally fall on a bell curve
  • Discuss how IQ tests may be used in schools

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