You mean, how do you force player characters to be adjusted toward the norms for their race? My question would be WHY do PC's need to be adjusted toward racial norms?
Simple: because that's what the race is. Thus, if you want to play an Elf you just have to accept the fact that Elves tend to be more dextrous than Humans and that this is reflected in a stat adjustment after you've assigned your stats, because when you're rolling up an Elf you're rolling up an Elf, not a Human. It's as much a part of the territory as accepting that the native language of an Elf is 99.9% likely going to be Elvish.
Now don't take this to mean I like the way D&D has gone about doing stat adjustments through the various editions; I don't, as IMO there's a better (though slightly more complicated) way to do it. See below.
The DM can and will assign stats to NON-player characters however they wish. If they want more elves with higher dexterity scores, doing more dexterity-oriented things then that is what the DM will have happen. But I don't see any reason that PC's need to ALSO noticeably reflect ability score patterns seen in their entire race. If a player wants a dexterous elf to reflect the trend seen in NON-player character elves, then let the player assign a higher amount to Dex, and a lower amount to a stat where elves tend to be lower than humans. If the player doesn't mind their PC being in the lower Dex percentile for their race, SO BE IT.
Yes, but what exactly does "the lower Dex percentile" represent when talking about an Elf? An Elf might, for example, be Dex 8 in Human (and thus game) terms but be Dex 6 in the eyes of other Elves.
You do not need to use stat bonuses/penalties to reinforce racial stereotypes that the DM is going to assign to NON-PLAYER-characters as they see fit anyway. Let the DM worry about the NPC's reflecting the idea that
in that campaign world Elves tend to be more dexterous. Let the PLAYERS decide what they want stats for their character to reflect. Period.
Keep in mind that when the player is rolling stats, the stats as rolled (or assigned) are for a Human. The 3-18 bell curve is based on Humans. Thus, if you're not rolling up a Human these stats won't be correct.
But those differences aren't hard-baked in the first place. NON-player demi-humans run the gamut of low to high in various stats just as humans do.
Agreed. BUT, the end points of those lows and highs will be different for each race.
For a particular stat there will be a higher percentage that are better or worse, but they aren't ALL better, nor ALL worse in the same way - nor should they be - NOR should we keep acting as if they are.
There will be exactly the same number who are better or worse than the racial average
for that race: it'll be very slightly less than 50% that are higher, the exact same number that are lower, and the remaining few are bang on it...an average that very well might not be the same as the 10.5 average for a Human. So, here's how to accomplish this:
First, as DM decide what the low point and high point for each stat for each race will be, relative to a Human. Carrying on with the same example: Elves are dextrous, so it's simple to assume the clumsiest Elf is still going to be somewhat more dextrous than the clumsiest Human - so in Human terms the low Elf dex is 6. However, the most dextrous of Elves can reach levels of grace beyond what a Human can do, so let's put their high to 19.
So now Elvish dex is on a 6-19 bell curve; and you can either find some combination of dice to roll this or, much easier, just design a table to convert each result on the standard 3-18 bell curve to its corresponding number on 6-19.
Like this table
right here. (if you scroll up from where this puts you you'll see the stat adjusts we use for each stat by race)
Note that sometimes stats get adjusted down - Part-Orc Charisma, for example, gets hammered. Hobbit and Gnome Strength doesn't do very well either. And so on.
Why does this need to be forcibly built into PLAYER character creation? Again, can't a player choose to create a NOT-above-human-averages-dexterity elf? Can't they choose to create a NOT-above-human-averages-constitution dwarf? If so, then let them.
Players can always do this, but they have to accept that both the system and the setting will more or less gently fight against them when they do.
If they want the "usually more dexterous" elf as their character, they can. They just assign a high score to Dex. If they want a "usually more high constitution" dwarf, they just assign a high score to Con.
Where instead they should be able to assign an average score to that stat and let the system convert it to whatever the average score would be for that race.
But can you argue that PLAYER characters MUST reflect those tendencies?
Yes I can, to the extent that if that's what the setting demands the player is somewhat obliged to go along with it. Sure a player can stick a 6 onto a Dwarf's Con score...but that's 6 on the Human bell curve; the same relative-to-average score for a Dwarf might be 9 or 10 in game terms.
Can't those tendencies be reflected by their choices in assigning stats as to whether their PC will demonstrate those tendencies or not?
Within race, sure. On the objective this-is-how-you-compare-to-a-Human scene, not so much.
I find it difficult to buy into that line of reasoning at all. If you take two characters with identical ability scores and hand them to two different players and say, "Your scores have been assigned. Make whatever character you like but there are no default RACIAL abilities," would you get all humans?
I think you'd get a lot of characters who played like Humans - even more so than now
If someone under those restrictions decides they DO want to play a demi-human despite not being bribed to do so by racial abilities within the game, wouldn't they still do so and still play their character AS a demi-human and not a human to at least SOME degree? I don't buy that ONLY mechanical benefits of a choice of race will produce results that don't seem like a re-fluffed human.
In fairness, you're making an assumption here for which I can't blame you as it's the way 5e is designed, but it's something I really don't like: that all racial abilities are benefits.
My philosophy runs more toward no benefit without penalty - it's easier to keep things balanced that way, and cut down on the arms-race aspect. Thus, if Elves are going to be on average more Dextrous then some other stat (in our game, Wisdom) is going to take a hit to compensate.