What Is… Intelligence Quotient (IQ)

Intelligence quotient: WAIS-IV test scales and bell curve scoring

In this series, I dig a little deeper into the meaning of psychology-related terms. This week’s term is intelligence quotient (IQ).

We’ve probably all heard of the IQ test as a standard way of measuring intelligence. What’s actually involved, though? Well, to begin, we should consider what intelligence is. There is no single agreed-upon definition, and there can be variations from one culture to the next. People have also proposed various other kinds of intelligence, including emotional intelligence.

Defining intelligence

A report by the American Psychological Association (Neisser et al., 1996) describes intelligence this way:

“Individuals differ from one another in their ability to understand complex ideas, to adapt effectively to the environment, to learn from experience, to engage in various forms of reasoning, to overcome obstacles by taking thought. Although these individual differences can be substantial, they are never entirely consistent: A given person’s intellectual performance will vary on different occasions, in different domains, as judged by different criteria. Concepts of ‘intelligence’ are attempts to clarify and organize this complex set of phenomena.”

Intelligence tests

A number of different psychometric tests are used to measure intelligence. One of the earliest was the Binet, which is still used today.

The Wechsler Intelligence Scale, which has both adult and child versions, is the most commonly used IQ test. David Wechsler, who published the scale in 1955, viewed intelligence as “the global capacity of a person to act purposefully, to think rationally, and to deal effectively with his environment” (as cited in Wikipedia).

The Wechsler Adult Intelligence Scale, currently in its 4th edition (WAIS-IV), is broken down into four key areas, each with several different types of tasks:

  • Verbal comprehension: similarities, vocabulary, general information/knowledge, comprehension
  • Perceptual reasoning: block design, matrix reasoning, visual puzzles, picture completion, figure weights
  • Working memory: digit span, arithmetic, letter-number sequencing
  • Processing speed: symbol search, coding, cancellation

An index score is determined for each of these four areas. Raw scores are compared to group norms for age and level of education and then an overall IQ score is determined.

IQ score distribution curve
IQ distribution curve – Dmcq [CC BY-SA 3.0]


IQ scores can range from 40 to 160. Scores are distributed on a bell curve. The median score is 100, and the majority of the population falls within one standard deviation (15 IQ points) above or below the norm. For any new versions of IQ tests that are released, scoring is normed to maintain a median score of 100.

To put this in non-statistical terms, an IQ score isn’t based on the percentage of questions you got right on the test. It’s all about how well you do in relation to other people.  Most people fall somewhere near the middle, and you have a small number of people at either extreme end.

Standard deviation is a statistical way of describing points along the bell curve.  As is shown in the diagram above, about 68% of people fall within one standard deviation (15 IQ points) above or below the median score of 100.

Let’s consider an arbitrary room with 100 random people in it. They’re lined up from lowest to highest IQ score. The median score, which falls between person #50 and #51, is going to be 100 IQ points. The 34 people to the left of person #50 are all going to fall somewhere between 85 and 100 IQ points. The 34 people to the right of person #51 are going to fall somewhere between 100 and 115. Fewer than 5 of the people in that room are going to have an IQ less than 70 or higher than 130.

Pattern over time

IQ test scores tend to be fairly consistent over time. A greater number of years of education is associated with a higher IQ score. Higher IQ is associated with greater success at school and better work performance. It’s unclear exactly what the role of heredity is, but there is a link between biological parents’ IQ and a child’s IQ.

I had an IQ test done when I was in elementary school, either grade 2 or grade 3. I’m not sure which specific test it was. I think the score I got then was probably pretty accurate, although the report that went along with it predicted I would have certain styles of learning, and that was not so accurate. I sometimes wonder what it would “feel like” to have a significantly different IQ.

Have you ever done an actual IQ test?


You may also be interested in the post How Much Does Intelligence Matter?

The Psychology Corner: Insights into psychology and psychological tests

The Psychology Corner has an overview of terms covered in the What Is… series, along with a collection of scientifically validated psychological tests.

Ashley L. Peterson headshot

Ashley L. Peterson


Ashley is a former mental health nurse and pharmacist and the author of four books.

36 thoughts on “What Is… Intelligence Quotient (IQ)”

  1. I’m always surprised how many people think they are above ‘the norm’ even when you explain the Bell curve.
    I’ve had those tests taken in school, I think I was 11 years old, to determine what choices you could make for further education. I came out as a good normal and didn’t end up a plumber, what was my career goal at the time.

      1. You wrote that the IQ is considered stable during time. Do you think that mental illness, like a depression, can alter that? As mental illnesses can change some things in the brain? And I see that memory is also a part of the total IQ, can mental illness have a (neg) effect then?
        My question is, I guess, I feel dumber, am I really?

  2. This is a fab overview! So 40-160, with 100 being median. I remember doing one online once a very long time ago, but probably when I had more faculties fully-functioning, and I’m pretty sure it came out poorly. Like, very, very poorly. As in, how am I even typing this with an IQ that low? 😂

  3. I had my IQ tested when I was a child and they wanted to judge something (probably development) from the stress of the foster hell (I guess). I had it tested again in 2012 when they were determining if I qualified for disability. It was higher than I thought it would be, but lower than what I thought too. It’s average. Nothing wrong with that. The odd thing is that a lot of people consider me ‘smart’ (which isn’t the same as being intelligent, but the two are often considered the same thing). I tell them I’m not ‘smart’, just have a lot of common sense. It’s true too.

    1. IQ seems like a strange factor to go into a decision on disability, unless the disability in question is intellectual. Smart people can be just as disabled as not smart people….

  4. Why would an IQ test have to be administered by a psychologist? It is no different from any other standardized test, SAT, GRE etc. When I was in school there two IQ tests used, the Stanford-Binet and another the name of which I cannot remember. On one the average was 100, with ‘genius’ being at 160, the other the average was 125, with ‘genius’ being at 175. On the first measure my IQ was 134, on the second measure my IQ was 166 – in any case all it proved was that I was good at taking tests –

    1. it’s fairly common for psychometric tests to have qualification levels required to administer and score the tests, and to even purchase them from the testing company.

      1. Administering the test involves handing out the test booklets/sheets, making sure everyone has a #2 pencil and that nobody cheats! For this you need a psychologist? There weren’t enough psychologists in NYS to administer the IQ tests for the entire city of NY LOL IQ tests were given city-wide on the same day. Same as state-wide reading/math tests and Regents exams. Teachers who handed out the tests were not the people who scored the tests.

  5. We had some variation of this among neuropsych testing done after a somewhat troubling concussion as an adult. The intelligence portion concluded no permanent cognitive damage and any deficits (pretty judgy word) were long-standing.

    We’ve always struggled with the spacial—build this drawing out of blocks. If you fold this paper at the perforations, what will it look like at the end? Fuck if we know…

    Somehow we were able to identify a “trellis“ by being shown a picture. Don’t know how the F we knew what that was. We did not know who Catherine the great was. Apparently that was among our “deficits.“ lol

  6. I haven’t had an IQ test, but I’m a great test-taker, or used to be. My ACT score qualifies me for membership in Mensa and even higher IQ groups than Mensa. I belong to Mensa for the social aspect, otherwise I would have no friends and never do anything. I can report that having a high IQ doesn’t necessarily mean you will be successful at work or relationships. It doesn’t mean you will be wealthy or belong to a particular political party. Etc. I’ve run into a lot of negativity when I mention my membership, but it’s the one group I’ve found post-divorce that’s full of various types of people at various levels of success who accept each other with all their various flaws. Except there is that pesky membership requirement.

  7. I did an IQ test when I was ten. It was advertised by Mensa in the newspaper. They would send you the test, you did it and returned it for marking. If you got over a certain mark, you could sit another test under formal exam conditions and if you got enough on that one you could join Mensa. I scored 150, which was enough to take it further, but I didn’t want to join Mensa, so I never bothered. My parents have the letter somewhere saying I’m in the top 1% of the population. (In Yiddish we call this nakhas, reflected glory from your children.)

    I know IQ is not supposed to change much over time, but like one of your other commenters, I feel much stupider than I used to be. I don’t know why – age, depression, length of time since formal school? My verbal reasoning is probably still good, but I feel that I am much less able to think logically and perform basic mathematical operations than I used to be. I’m not sure how easily I could solve a quadratic equation any more. I’m not sure I can even remember what a quadratic equation is any more…

  8. My younger brother and partner were classified as gifted, while I wasn’t. So o don’t really know what’s my IQ though my partner and chosen family tell me the IQ number isn’t that important.

      1. Yup, it’s part of a process called “streaming” where you’re funneled into different educational bands according to your supposed intelligence and academic results.

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