It had generally been said in my gaming circles that Intelligence (INT) in D&D was the equivalent of an intelligence quotient (IQ) divided by 10. Or INT * 10 = IQ. The average IQ is 100, and the D&D manuals stated that the average INT was 10 (really 10.5 by rolling 3d6). Heroes, rolling 4d6 and dropping the lowest, have a mode of 13 and a mean of 12.2446. The whole INT distribution, however, does not fit as well as its mean onto the real IQ distribution.
The old Stanford Binet calculated IQ as 100 times the ratio between a child's age and the age of children with the same performance on average on particular tasks. So, a 6-year old who performed as well as the average 9-year old would have an IQ of 150. This method of determining IQ scores becomes less useful as children grow up, and makes no sense to apply to adults. The newer Wechsler tests calculate IQ by taking the scores of everyone in an age group and putting them on a normal distribution with a mean of 100 and a standard deviation (SD) of 15. This is a much more clear way to evaluate how a person functions intellectually compared to same-age peers.
If we take the distribution of 3d6 rolls and associate the probability of each sum with the prevalence of corresponding IQs in the real world, we can get a better impression of what a D&D INT score means in terms of IQ. If we say that 3d6 simulates a normal distribution of intelligence, then here are the translations (rounded) of each INT score into an IQ score:
INT IQ
3 57
4 66
5 72
6 78
7 83
8 88
9 93
10 98
11 102
12 107
13 112
14 117
15 122
16 128
17 134
18 143
The IQ scores in this table are calculated from the z-scores for the middles of each range of probability that maps onto each INT score. For example, an INT of 3 and of 18 each have a 0.463 chance of occurring (1 in 216, which is 6^3), but I cannot use the z-score for .00463 to calculate IQ for an INT of 3 and work my way up because I would be left trying to use the z-score for 1 with the INT of 18, which is infinity standard deviations. Also, I would end up using the z-score for .5 for an INT of 10 when it truly applies to an INT of 10.5. What works is to use the z-scores for the midpoints between probabilities. So, an IQ of 57 (2.84 standard deviations below the mean) corresponds to the z-score for .0023 (the midpoint between 1/216 and 0, since it is impossible to roll less than 3) with some rounding. The probability of rolling at least a 9 is .375, and the probability of rolling at least a 10 is .5, the midpoint is .4375 for an INT of 10, which has a z-score of
about -.157. 100 (the mean) minus (.157 * 15 (the standard deviation)) equals about 97.645, so the IQ for an INT of 10 is 98 after rounding. Each IQ score was calculated this way.
As you can see, multiplying INT by 10 does not give you your character's IQ. Now, this does not address differences in intellectual ability by age group. A stereotypical D&D party consists of characters emerging from adolescence into adulthood, roughly in the same stage of development, so it should be okay to use this scale. A 12-year old with a 120 IQ still lacks much of the functioning of a 20-year old with a 100 IQ, and a 30-year old with a 100 IQ functions a little differently than a 50-year old with a 100 IQ. Age tends to result in a larger vocabulary and body of knowledge with a slower information processing speed, but I may go deeper into how RPGs simulate the effects of aging on intelligence in later posts.
For that matter, using an overall IQ score obfuscates the differences in each person's separate areas of intellectual functioning. "Intelligence" refers to many different abilities that we have, and a dozen people with the same IQ can each function differently from each other on a variety of tasks. Few simulation systems break down intelligence into components, and that is also a topic I will return to in future posts.
