Caliban said:
It's actually in the DMG, not the PHB. On page 23.
I've always hated that table. It basically says "stupid races can't ever be smart."
Say your race has an avg Int of 9. Not too bad. Same as a half orc right? Nope sorry according to the table if you put an 18 in Int then you have a 14. If you put in a 16 you have a 12. Somehow you've got -4 now.
It only gets worse, with Int 6-7 (technically a -4), if you put in an 18 you get a 10. Put in a 16 an 8 (both a -8). How about 15 or 14 also an 8 (you figure it out).
It seems to usually double the penalty, but theres no real reason behind it as far as I can tell (other than to say All Trolls are Stupid, no really ALL of Them).