What metric could be used to convert a mental stat between 3.5 D&D and GURPS?

Question

When converting character back and forth between 3.5 D&D and GURPS, there are a few stats that are easier than others.

As an example, an easy stat such as Strength could be converted by using the weight load values using the metric of weight to find the equivalent values, or possibly by using the metric of damage dealt to find equivalent Strength values.

Non-physical stats are more troublesome as they do not always have numerical values that map well to each other.

What metric or method could be used to compare and convert a mental stat, such as INT from D&D and IQ from GURPS?

Answer 1

There is a detailed D&D conversion page in the GURPS wiki, and it suggests simply using the INT score as the GURPS IQ score.

This agrees with the original sense in which the INT stat was conceived in D&D. The average of 3d6 is about 10 and this was considered roughly as the average IQ of 100. Based on this, an article by Brian Blume in Dragon magazine #8, "So, You Want Realism in D&D?" suggested dividing IQ test scores by 10 to obtain INT scores in D&D. You can check various characters from GURPS Who's Who and this scale factor also seems to align for them. For example Einstein is stated to have a GURPS IQ of 15 and most websites claim that he had 160+ IQ for him. (Well, the stand deviation of 3d6 is about 3 and the standard deviation for real-life IQ is calibrated at 15, so the divide-by-10 rule is likely to become cruder at the extremes, but all these are fun approximations anyway.)

Answer 2

While IQ and other attributes in GURPS and D&D 3.5 in theory average about 10 for humans, the ranges (and meanings) of values further away from 10 are different.

For one thing, in GURPS, every single point is significant, giving a +1 or -1 to skill levels and the actual numbers rolled against for all spells, related skills and defaults, and challenges (including perception rolls, recovery from Mental Stun, resistance to many types of manipulation and magic, etc). In D&D 3.5, the die-roll modifier is only 1 per 2 levels of INT, and the roll is made against a D20 instead of 3d6, so +2 D&D INT increases your chances by 5%, but even +1 GURPS IQ can increase your chances by as much as 12.5% (or less depending on where it falls on the bell curve).

For another thing, a 3d6 roll will about 9% of the time give you a GURPS IQ equal or better to Einstein (15+), and about 16% of the time (1/6) give you a GURPS IQ that is lower than normal human IQ (7 or less). And published D&D characters also have a wider range than you'd tend to see in typical GURPS books.

IQ 8 is the lowest normal GURPS IQ, so I'd tend to set a converted character's IQ to at least 8 unless it's described as mentally handicapped.

In general, I tend to set GURPS IQ to roughly half the distance from 9 that a D&D character is, with an 18 INT perhaps being a 15 GURPS IQ. I also look at what the text description is like, and consider representing some of their abilities with high mental skill levels and/or GURPS mental advantages such as Magery or various types of Aptitude, Talent, or Eidetic Memory, and/or counter-balancing a very high IQ with some mental disadvantages that limit the effects of very high IQ.

So I typically do something like:

Int	IQ	Comment
3	6	(lower if cripplingly mentally disabled)
4	6-7
5	7	(noticeably mentally challenged)
6	8
7	8
8	8-9
9	9-10
10	10
11	10-11
12	11
13	12
14	12-13
15	13
16	13-14
17	14
18	14-15	possibly (rarely) higher.

(+/- 1 if it seems to fit the description, possibly trading IQ for mental ads/disads in some cases)