First a somewhat long side-note/introduction/rambling to explain why I'm House-Ruling those Races in the first place as well as my thoughts on how I came to this idea. If you just want the House Rule simply go to the end of this post, really.
So I have been running my D&D campaign for many years now, using the 'Castles & Crusades' system. As far as OSR systems go, it's my favorite.
It was the one that first used the OGL to return to a more Old-School style. It was the first to use the Saving Throw as a skill check. (Attribute checks basically do almost everything in the game in fact.) It involved every single Attribute to specific saving throws, making no attribute a bad choice. It's super easy to use with pretty much everything OGL/D&D out there.
It does a lot for me as a DM. I don't have to worry about skill points for NPCs. It separates Race and Class which I personally prefer. It doesn't make starting PCs a bit too powerful for my taste (Looking at you, 5e. Which to be fair I would have bought if C&C didn't exist already, it really is a nitpick.) It doesn't have those Level Limits I hate so much. (Although they forgot about the implications of removing said limits when it comes to Elves and the Wish spell!)
I know some people dismiss it because the SIEGE engine is often criticized. I myself am not an adherent of the SIEGE engine, where all Saves start at 18 except for Prime Attributes (all characters get to choose 2, except Humans who get 3) that start at 12. Instead of the saving throw Target Numbers going down as characters level up, players get to add their level to their die roll. It's pretty much the same thing really, except the numbers on the Target Numbers never change that way. As you can imagine, saving throws for Level One PCs on non-primes are kinda tough since you need 18 or higher to succeed. Since you get to add your level even at Level one I guess one could say it's 17 but still pretty tough, and that's for most of your saves unless you're a Human, then that's 50% of your saves. On top of that, for years I thought that if you were rolling a saving throw against a monster's special attack, its HD was a penalty to your roll, although today I was trying to find that rule somewhere and couldn't find it, so maybe I got it confused with the rules for Turning Undead and for Saving Throws vs Spells. I digress big time.
Anyway, I was turned off by the SIEGE Engine because it made Saving Throws a bit too harsh in my opinion. I decided to go the route of other OSR games, set the initial TN at 15 for all Attributes and then lower them by 1 every level. Simple, yet it removed the advantage of playing a Human and to some degree a Half-Elf raised by Humans in the game.
My solution was the use the rule of Basic Fantasy Roleplaying and give Humans a 10% bonus to XP (5% for Half-Elves raised by Humans.) Except I recently read this. My initial reaction was to consider increasing the % bonus to XP, but I figured now might be the time to get rid of the extra math while calculating XP for every PC and over the years I've noticed that playing a Human was slightly boring. Why not fix that and go back to what C&C was doing to an extent without using the whole 18/12 system?
So basically Humans get a +6 to one of their saves over other Races, and Half-Elves raised by Humans get a +2. If the initial TN is 15, putting a +6 on a Saving Throw roll seems a bit too good. Spreading the +6 as a +1 for every Attribute felt m'eh. So I could split the +6 bonus into two bonuses of +3 for Humans. But I was reminded again of how BORING it is to play a Human in many OSR games. I wanted something that would make it feel fun, even if in a very superficial way.
5e has the advantage idea of rolling two d20s and picking the best one as your roll, but an advantage won't allow you in any case to go over a TN you previously couldn't beat and I already use the advantage idea when I don't feel like coming up with a proper numerical bonus to something. It just wouldn't feel like this special thing only Humans have. Still, rolling more dice is fun, so maybe I can take that +3 and replace it with 1d6. On average it's the same thing, but it's more exciting.
So Humans have Human Spirit: 1d6, where they can choose two Attributes that will get a +1d6 bonus every time they roll them. And following that premise, Half-Elves raised by Humans get Human Spirit: 1d4 for one Attribute. When rolling with Advantage, only the d20s are multiplied, not your Human Spirit die.
Please let me know what you think!