Birthright does something interesting with humans. It divides them up into cultural groups and gives them bonuses as though they were separate races. The bonus modifiers for the Cerilians are the following:
Anuirean, +1 Wis, -1 Dex
Brechtian, +1 Dex, -1 Wis
Khinasi, +1 Int, -1 Con
Rjurik, +1 Con, -1 Cha
Vos, +1 Str, -1 Int
On the one hand, I really like this. On the other, I have some reservations about it. These single point perks and flaws are meant (I'm sure) to represent the various inclinations, cultural leanings, values, and training amongst those populations. That's neat, because cultural norms SHOULD be reflected in the game.
And yet... cultural norms shouldn't, to my mind, be as decisive a factor as the blatantly inhuman nature of demihumans. That +1 Dex, -1 Wis is no different from the dwarven alteration from the PHB (though to be fair, in Cerilia, dwarves get a +2, Con, -2 Dex which sort of addresses the problem). I almost want to adopt the more dramatic adjustments from Birthright for my own demihumans (almost, but won't).
I really like cultures and subraces. I really like stat adjustments. I really like when they work together. I think Birthright, alone amongst the AD&D supplements, solves this issue in the best way (eg, exaggerating the inhuman qualities of demihumans). Things such as the Complete Book of Dwarves tend to blend the idea of distinct subraces and distinct cultures—in fact, the notion of non-cultural subraces is something that might not make sense, though that's a debate about genetics in a primarily fantasy setting which is a road we probably don't want to go down.