Occasionally, D&D enthusiasts will discuss or debate how the game ability "Intelligence" score properly relates to the real-world measurement of "Intelligence Quotient" (IQ). This almost always spawns a heated debate, in large part due to the controversy over IQ-scores general (what it seeks to measure, how valid the measurement is, whether testing procedures are fair, how the tests have changed over time, etc.), and aggravated as some find it problematic to measure IQ in fantasy non-humans, animals, and monsters.
The original AD&D ruleset actually addressed this in the core rulebooks. Each of the AD&D 1st Ed. Monster Manuals included the assertion that "Intelligence indicates the basic equivalent of human 'IQ'" (MM p. 6, FF p. 7, MM2 p. 6, Deities & Demigods p. 6). Some slight restatement of this assertion appears in the later references; the DDG page adds a clause that the ratings specifically do apply "in monsters", while the FF page adds the parenthetical note "at least in concept even if IQ itself appears now to be much disgraced". Presumably this equivalence would be for intended for adult IQ scores.
Granted that the original designer of D&D (Gary E. Gygax, who created the various ability scores such as Intelligence in the first place, and wrote the original Monster Manual) specified that Intelligence does indicate a certain IQ rating, and that 10 is an average human D&D Intelligence, while 100 is an average real-world IQ, the simplest relation is to assume that Int = IQ / 10. This assumption is in fact borne out by the existence of a somewhat light-hearted article published in Dragon Magazine, issue #8, for converting real-life players' characteristics into D&D statistics, which asserts: "To determine your intelligence, look up the results of the most recent IQ test you have taken and divide the result by ten. This number is your intelligence rating." (Dragon #8, "So, You Want Realism in D&D?", by Brian Blume).
Finally, the same principle has been upheld for the most recent edition of the game. The Official D&D FAQ says this: "A character with an Intelligence score of 3 is smarter than most animals, but only barely... Ten points of IQ per point of Intelligence is a good rule of thumb, so your example character has an IQ of about 30." (D&D FAQ, Version 3.5; Update Version 09/28/05, p. 2).
Some D&D players are never satisfied with a rule as simple as the preceding. One of the most popular alternate theories by D&D gamers who are unaware of this history is to theorize that they should "compare the bell curves". That is, they contend that one should calculate a relation by considering what percentage of real-world people have a certain IQ range (constructed specifically with a 100 mean, and standard deviation of 16), and map that to a range of equal percentage likelihood when rolling 3 six-sided dice (range of 3-18, with a 10.5 mean, and standard deviation of 2.95). The end result is a formula such as Int = (IQ - 100) / 16 * 2.95 + 10.5.
A relation like this has the effect of scaling the extremes of IQ scores further out on the Int scale. This presents several practical problems: (1) animal-level intelligence would correspond to Int 1 = IQ 48 or Int 2 = IQ 53, which would be high enough to learn language; (2) the minimum for humans, Int 3 = IQ 60, is sufficiently high as to entirely miss several categories of real-world intelligence deficiencies (see below); and (3) the maximum for humans, Int 18 = IQ 141, is actually far below the results for some real-world people on standardized IQ tests.
The best example of this last problem is the Guinness record-holder for Highest IQ, Marilyn vos Savant, who reportedly has an IQ score of 228 (subject to some debate; see links at end). Under the "compare the bell curves" theory, this would translate to Int 34, which is wildly beyond the range possible in D&D by rolling 3d6 (in fact, beyond the range of most gods in D&D). Even if we are skeptical of this IQ score, considering the previous record holder's IQ of 196 results in Int 28, again far beyond the 3-18 result achievable in D&D. In contrast, the simpler linear relation properly brings these scores into the more reasonable range of 19 and 22, which is in fact naturally achievable in D&D via several methods. (E.g., a roll of 18 plus a few age or level-based ability bonuses, reasonable point buy, etc. The "compare the bell curves" method is not remotely correctable even by maximizing such increases.)
One amusing "advantage" of the scaled curve system is that it strokes D&D players' egos by making them look extremely smart in game terms. I've seen multiple online discussions of this topic in which everyone participating gleefully points out that their IQ scores translate to a D&D Intelligence of 18 or more under this model, and feel that that's entirely reasonable.
The simplest response to the "compare the bell curves" theory is that there's no necessity for the fantasy population of a D&D world to exhibit the same deviation (or mean) in intelligence as real-world humans. In fact, there's no strict requirement that the fantasy population actually matches the bell curve of a 3d6 random variable. It may in fact be a good idea to use a character-generation method that creates interesting, outside-the-norm, exceptional characters with greater frequency than exists in the actual population.
Another alternate theory is that IQ shouldn't be directly related to Intelligence at all, but rather that a formula should be generated that combines all the D&D mental abilities (Intelligence, Wisdom, and Charisma), weighting them in some fashion to generate the IQ score. This can be discounted mostly as a frustrated response to not having a clear real-world measurement or translation for the Wisdom or Charisma abilities. Clearly: (a) "Intelligence Quotient" is by definition a measurement of "Intelligence", and (b) the original designer comments (above) explicitly indicated an IQ:Int relationship and nothing else.
Int and IQ Categories
In each of the original AD&D monster books, a table of descriptive Intelligence categories was included, reproduced below. Among other details, note that Int 10 has always indicated "average (human) intelligence", while the maximum natural score of Int 18 is considered a "genius", and anything beyond that is apparently extra-human, titled "supra-genius" or "godlike".
|0||Non-intelligent or not ratable|
|8-10||Average (human) intelligence|
IQ ratings, however they are tested or generated, are scaled so that a score of 100 is average for a person taking the test (in a particular age category). The original IQ classifications by Terman are reproduced below (taken from http://members.shaw.ca/delajara/IQBasics.html ). A few similarities can be noted to the table of AD&D Intelligence above: (1) the Int ratings usually go up by factors of 2 points, while the IQ categories usually rise in steps of 20 points, (2) at the upper end, an Int of 17-18 indicates a "genius", while a 170-180 IQ would also be categorized as "genius". This strongly indicates that the D&D system was in fact designed with an eye towards a x10 translation between IQ and Intelligence.
|90-110||Normal or average intelligence|
|120-140||Very superior intelligence|
|140+||Genius or near genius|
Links on IQ
Overviews of intelligence testing:
Criticisms of IQ scores and their usage:
Institutes that still prefer the Stanford-Binet IQ scale for gifted students:
Estimated IQs of historical geniuses:
Debate on Marilyn vos Savant's IQ score:
The overall rise in IQ scores since 1920: