What is IQ?

Here is the funny thing: IQ is being used to rate, admit, qualify, and select us all the time, but nobody knows the answer to the title question. Nobody knows what the psycho-neural substrate that determines IQ is.

That fact leads some people (including some very high IQ people) to deny that IQ is a meaningful concept. That view is hard to defend in the face of conclusive evidence that whatever IQ is, (1)It can be measured, (2)It is reproducible and stable throughout adulthood, (3)It is highly predictive of accomplishment in a wide variety of skills, from engineer to manager to football quarterback to infantryman.

The concept was invented to determine degree of mental retardation in school children, but its use has spread throughout society. I am a bit embarrassed to admit that my own children were IQ tested before their fourth birthdays (an admission requirement for a certain preschool). Most American children are IQ tested in school, and employers from WalMart to Microsoft to the National Football League test prospective employees. The military pioneered this kind of testing, and it spread through the world because the military found that it worked - it allowed them to identify what recruits could be trained to do its various jobs.

I seem to recall that a certain blogger who has expressed doubts about the IQ concept was admitted to two of the most elite educational institutions in the world - quite likely the two very most elite such. I am quite certain that he had to pass difficult exams, exams with a high g-loading (IQ component) in order to be so admitted.

For reasons that I consider mainly political, the SAT and GRE no longer admit to being IQ tests, but in fact they are, at least in part. So what is it that IQ tests test anyway? The most popular IQ test today, the WAIS-III, has the following components, according to Wikipedia:

14 subtests of the WAIS-III

[edit] Verbal Subtests
Information
Degree of general information acquired from culture (e.g. Who is the president of Russia?)


Comprehension
Ability to deal with abstract social conventions, rules and expressions (e.g. What does "Kill 2 birds with 1 stone" metaphorically mean?)


Arithmetic
Concentration while manipulating mental mathematical problems (e.g. How many 45c. stamps can you buy for a dollar?)


Similarities
Abstract verbal reasoning (e.g. In what way are an apple and a pear alike?)


Vocabulary
The degree to which one has learned, been able to comprehend and verbally express vocabulary (e.g. What is a guitar?)


Digit span
attention/concentration (e.g. Digits forward: 123, Digits backward 321.)


Letter-Number Sequencing
attention and working memory (e.g. Given Q1B3J2, place the numbers in numerical order and then the letters in alphabetical order)

[edit] Performance Subtests


Picture Completion
Ability to quickly perceive visual details


Digit Symbol - Coding
Visual-motor coordination, motor and mental speed


Block Design
Spatial perception, visual abstract processing & problem solving


Matrix Reasoning
Nonverbal abstract problem solving, inductive reasoning, spatial reasoning


Picture Arrangement
Logical/sequential reasoning, social insight


Symbol Search
Visual perception, speed


Object Assembly
Visual analysis, synthesis, and construction


Optional post-tests include Digit Symbol - Incidental Learning and Digit Symbol - Free Recall.

Quite a variety of things seem to being tested, but there are several main subthemes: speed of mental processing, knowledge, memory, pattern recognition, and processing information held in memory. One of the striking things, the thing that really makes the concept of IQ meaningful, is that these rather different seeming skills are usually - but not invariably - highly correlated.

Interestingly enough, ageing affects them differently. Speed of mental processing declines markedly with age, and so do some kinds of memory, but other facets of reasoning seem more durable.

IQ test score are based on a normal (bell-shaped) curve, usually with 15 points equally 1 standard deviation (1 point = 1/15 standard deviation). Scores more than two standard deviations above or below the mean = 100 are probably not especially reliable.

Older forms of the IQ test were based on an age ratio - thus Marilyn vos Savant, who at ten tested at a mental age of 23, was judged to have a record IQ of 230 or so. While doubtless she was a very bright ten year old, such a score would be impossible on a normed, standard deviation based test.

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