Ogrork the Mighty said:
Then how do you rationalize 1-in-216 people having a stat of 3? Isn't that a bit high?
Not according to the 3d6 method of rolling (which just goes to show its weaknesses in regards to emulating real life). Besides, if you look at modern statistics you will be surprised at how many people are considered mentally or physically disabled. Recall that 1/216 is just shy of half of one percent.
Recent disability statistics for Japan, for instance, gives us the following:
Motor dysfunction (limb and body) 1,657,000 (56,5%)*
Visual disability 305,000 (10,4%)*
Hearing and speech disability 350,000 (11.9%)*
Internal organ disorder 621,000 (21,2%)*
Total 2,933,000
* (of the Total number of disabled: 2,933,000)
This is about 1% of thier population - and it is only for physical disabilities. Even if you ignore the visual / hearing, you still have organ disability (Con) equal to ~0.212% and limb / body disabilities (not necessarily just low str, which may or may not be considered a disability) at ~0.565%. If you consider the fact that Str and Dex problems are more or less lumped together, then the ~0.565% is suddenly halves to ~0.2825% Str, ~0.2825% Dex, and ~0.212% Con disability / severe penalty / etc.
Statistics for mental retardation / disability suggest a somewhat different view, however:
Mentally retarded persons 390,000
Mentally disabled persons 1,570,000
Total: 1,960,000
Mental Retardation, if we view this as extremely low Int, suggests only half as many as would be expected using the 3d6 system - if not less. Mental disabilities, on the other hand, are extremely high. However, if we take the total and divide by three (assuming retardation is but one factor of unusually low Int, and assuming low Int/Wis/Cha are equally divided among the population), then we find about .22% of the population suffers from either low Int, low Wis, or low Cha. This is -as with physical disabilities - half the expected score.
~0.25% vs half a percent. That suggests that - at worse - the extremes are off by a factor of two. Not bad, considering. More than close enough for most RPG work. So, perhaps it should be:
03,18 . . . 0.00231 (01/216 * .5000)
04,17 . . . 0.00992 (03/216 * .7143)
05,16 . . . 0.02579 (06/216 * .9286)
06,15 . . . 0.05291 (10/216 * 1.143)
07,14 . . . 0.09424 (15/216 * 1.357)
08,13 . . . 0.15278 (21/216 * 1.571)
09,12 . . . 0.20668 (25/216 * 1.786)
10,11 . . . 0.25000 (27/216 * 2.000)
So, 50% of the population is average in at least one stat, and 0.002% of the population (1 out of ~500) is phenomenally low or high in one stat. That sounds about right to me.
Granted, I only used statistics from one country, but it was the first that came up when I googled for such. I will also note that most disabilities might fall under higher stats than a '3'. Mental retardation, for instance, has several grades of variance. Some might easily be considered a 6 or 7, while others would likely be considered a 3. But they are all grouped together in the statistics above, which suggests that lower scores are even less common than the ~1/500 suggested above, and mid-level scores are even more common.
Perhaps, instead of varying from 0.5 for lower scores to 2.0 for higher scores, I should have used 0.25 for lower scores and 4.0 for higher scores. There is not enough data to be certain if this would be more accurate or not, however. In the end, it is mostly guess work, and I think the scheme I wrote above is accurate enough for most suppositions and approximations for RPG worlds.
Of course, trying to use real world statistics for something as relative as RPG is likely an act of futility.

But it does suggest a more moderate view than 3d6 would typically allow.