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.
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.
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 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?
- Neisser, U., et al. (1996). Intelligence: Knowns and unknowns. American Psychologist, 51(2), 77-101.
- Wikipedia: Intelligence quotient | Wechsler Adult Intelligence Scale
The Psychology Corner has an overview of terms covered in the What Is… series, along with a collection of scientifically validated psychological tests.