I think the core problem to this question is really "what do the mental stats actually mean?"
The physical stats are pretty easy, an 18 Strength means you are the strongest man in town and can lift a ton of stuff. There are even charts for how much you can lift.
Mental stats?
3x SRD defined them pretty well, but what is the difference between 12 points and 14 points?
For Int, the assumption has always been that 1 point of INT = 10 points of IQ. Okay..... nice to know. But what does that translate into what the character can do?
Then to make it worse, game mechanics take it farther than the above definitions do, with the age old entangling of Charisma and Comeliness... and people being who they are, its easier to say a high Charisma makes the PC prettier. The PHB descriptions are better, but still don't give you a gradient.
I tried, but my google-fu failed me. I recall sometime in the hazy past a chart of the mental stats that had key markers at specific levels that indicate how to roleplay that score.
Something like; Int:
1: low animal intelligence, respond to instinct and urges. Cannot learn from past mistakes, no tactics other than rush in and fight to the death.
3: high animal intelligence, responds to instincts but can learn or be trained. Rudimentary tactics and ambushed.
5: Damaged or immature human, reacts instinctively, cannot learn to read and is limited to simple communication. Occasionally self-destructive due to lack of comprehension.
8: uneducated human, was not exposed to higher concepts or critical thinking skills. Tends to follow others.
etc...
{edit}
I found a portion, from the 1e AD&D monster manual lists the INT score, but it doesn't talk about roleplaying markers..
[sblock]
0 Non-intelligent or not ratable
1 Animal intelligence
2-4 Semi-intelligent
5-7 Low intelligence
8-10 Average (human) intelligence
11-12 Very intelligent
13-14 Highly intelligent
15-16 Exceptionally intelligent
17-18 Genius
19-20 Supra-genius
21+ Godlike intelligence
[/sblock]