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What your intelligence score means:
I have always been in the camp that your intelligence score is roughly equivalent to your IQ divided by 10. One person (sorry, can't remember who and too lazy to look it up, but credit!) suggested something like your modifier times 10 plus 100. I have to say I'm finding a certain appeal to that process since it would allow my to personally justify a larger range of intelligence scores being in the "normal" range.
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It was actually me who suggested it,
post #97 upthread, but I can't believe no-one's thought of it before.
I reckon that if the general population of NPCs is represented by 3d6 (which is an assumption) the standard deviation of their Int scores will be close to 3 (mathematically, it's actually sqrt(35)/2). We want to map that onto an IQ statistic with a standard deviation of 15 or 16, so we need a factor of about 5, then adjust to make the average come out right.
So we could take IQ = Int*5 + 47.5 but that's a bit fiddly and hard to remember. Approximating it as 5*(Int+10) is easier, but as in 5e we already have Int modifiers worked out, 10*IntModifier + 100 is even easier and gives the same result (albeit with less precision).
3d6 is a fair approximation to a normal distribution, although the extremes are chopped off. An Int of 17 or 18 will turn up 4/216 times, or about 2%. That's a Mensa level intelligence - the top 2% of the population, an IQ of 130 or 132 (depending on the IQ test used). Using 5*(Int+10), Int 17 works out as 135, which is pretty close to that.
Point buy skews the PC to have good scores - PCs are supposed to be outstanding individuals and the statistical distribution of their ability scores isn't supposed to be the same as the general population. Even so, you can't get a human above Int 16 with point buy, and on my scale that's IQ 130, or just hovering on Mensa level, which seems fair. To get higher, you need a feat or level advancement and that seems fair too.
Of course, all this begs the question of what Intelligence
means 