I am working on the Lore for my setting, and part of that is fleshing out some of the ways the different races live and interact with the wider world.

My Dwarves have a special culture of tracing their ancestry to make sure their gene pool remains strong.  A mother dwarf makes a tapestry for her children depicting their genealogy so they have something like an in-depth birth certificate.

My Orcs have a culture where they have the tribes get together in the Spring and this is when mates are selected.  Each family has a rotation of other clans they meet up with through the years.  This helps them avoid weak genes from getting too much help.

But other than gene pool things, I am considering how living a longer life than a typical human causes social strife.  An Half-Elf  has one parent that will die typically before them and one that will live long after them.  If they grow up in a human-centric community, most of their friends will die long before they do.  If they mate with a human, which would be very likely, they should outlive their mate, and their own children.  It seems this would cause many Half-Elves to become melancholy and even a bit reclusive when the waves of death among their childhood friends hit.

And then there is the Elf parent.  What does the Elf parent do when their Half-Elf child(ren) are grown but their spouse has passed?  They have an extremely long life ahead after their spouse dies.  Would they tend to return to the Elf community?  Would they remarry?  Would they consider marrying a human a second time having seen how it is likely to play out again?  I am going to see if I can flesh out some of the emotions in all this and then figure out how a culture would evolve in those circumstances.

If you've never read any of the Dragonlance novels, this is one of the major ingredients in the stew that is Tanis Halfelven's angst.  I want to say he meets a more well-adjusted half-elf, but I might be making that up.

There's also the Song and Sword books by Elaine Cunningham; one of the main characters of several is half Moon Elf in the Forgotten Realms and very strongly identifies with that side of her heritage, which is not widely reciprocated.

I'd say almost all halfelves have no idea who their father is. Specifically. 

Imagine being a female elf. Would you risk siring a child with a lifespan only a fraction of your own?

So. Fathers. And elven males don't fall for human women. They might fancy them that one night at the tavern, after one too many flagons of wine - but in the morning they move on, back to the forests, and never thing of that comely serving maid ever again. 

At this point there have been so many half-elves, and many of them must have mated with each other, that aren’t they their own species by now? I mean otherwise they wouldn’t actually be their own species.

As I have been reading this, I have realized that half elves, half orcs, and other things like that, imply that humans were naming the race of people. Do half-elves and half-orcs call themselves different things? If an orc and elf mate, is the result a Half? I'm just making things up at this point. Half-humans would be half human and half ???

Yes, previous editions actually had half-elves sometimes get referred to as "half-humans" by elves.

Eliminating the Half=Elf and the Half-Orc from the 2024 PHB did a great disservice to the past lore, IMO.  Tanis half-elven has to be rolling in his grave in the Dragon Lance setting.  The angst and struggle in the novels is why we loved the character and others that struggles with their Heritage. He rose above it.  I believe WoTC dropped the ball and turned their backs on all of the old books and novels.  I looked around for answers why the half races were eliminated.  And I read one answer was that WoTC thought that having half races was racist !  It's sad that our favorite game is becoming PC  (politically correct).  Message to WOtC.....DnD has racism.  Dwarves hate Orcs, Elves hate Orcs.  Most races fear and hate Dark Elves.  Only recently with the hard work of Drizzt novels, that story line has shown the capability to overcome racism. There are exceptions that show hope and growth

I have a suggestion for WoTC,  since PHB 2024 is already out, add the half races and needed character info into the 2024 Monster Manual.  Bring back or beloved chararcters and their history. thanks

The problem with saying “half elves represent X social experience” in the current model is that they’re actively working to avoid trying to set those dynamics as fixed points in the core books; you will note that despite your assertions on how things simply are across the body of D&D, they say nothing about how the different races get along. And no, this does not mean FR lore is being cancelled, just that it’s not appearing as the implicit if not explicit default in the updated PHB. There are pros and cons to this approach- overall the new PHB is painfully short on more than a few general roleplaying prompts- but it’s not the end of the world. The experience of someone of mixed ethnic heritage is extremely complex and varied, so from the narrative side it’s hard to write a description that doesn’t seem to pigeonhole them, and there’s also a whole mess of issues and arbitrary limitations around the part where only humans can pair up with another race and have children, particularly when only two other races are then compatible with them. Ideally the upcoming DMG will have the section on “any pairing can have kids with a DM thumbs up, average the lifespan, pick one block from the parents for traits, and appearance is whatever the player and DM agree on”; it’s much better at accommodating the breadth of possibilities people come up with rather than only supporting a tiny percentage of them. The present half elf block also is not only still supported on the character builder, but its source is still available for purchase despite being legacy content, so there has literally been no loss or removal at this point. Personally I would like to see the block come back eventually- shifting it from explicitly half elf to a lineage similar to Ravenloft for “mundane individual manifesting latent fantastic traits” would not only preserve the spirit but broaden the range of uses. But, regardless of what future developments come, this has absolutely no impact on existing or future narratives and minimal impact on gameplay options.

Let’s turn some of this around. Most elves and humans never leave their home communities so they don’t normally mix. The ones who do mix are mostly adventurers or traders. Both of whom can’t count on living their full potential life spans because of the dangers of their professions. Under those conditions romances between folks that depend on each other for survival aren’t surprising and such romances will provide offspring. If the offspring are lucky both parents survive till they are grown and out. If not they may be orphans or half orphans. Even elves with their long lives  can’t count on being around for long periods after a partner dies s it might not matter. Half elves born of these conditions are likely to have a variety of traumas to deal with but that’s after the fact of growing up. 
half orcs, following the old lore, are typically the byblows of war captures or occupations. And the fathers probably don’t care much about them while the mothers may or may not care given the processes causing the births. So a probable different set of traumas for the byblows. 
This should give those that love complex traumatic backstories plenty of space to work. On the other hand there should be enough retired (permanently or temporarily) adventurers etc that manage mixed marriages long enough to lovingly raise mixed bloods in fairly happy homes for those of us that want simpler or happier backstories. 

Quote from Wi1dBi11 >>

Let’s turn some of this around. Most elves and humans never leave their home communities so they don’t normally mix. The ones who do mix are mostly adventurers or traders. [...]

 This makes assumptions about the nature of the world and the cultures involved. IIRC Eberron has enough human-elves that they're considered a separate species. In any sufficiently cosmopolitan setting (which ought to include the Forgotten Realms, or at least the most-portrayed parts), they'll probably be not particularly unusual.

Also, assuming PCs are more or less a representative sample of the population, they ought to be incredibly common in most D&D worlds. :)

Quote from jl8e >>

Quote from Wi1dBi11 >>

Let’s turn some of this around. Most elves and humans never leave their home communities so they don’t normally mix. The ones who do mix are mostly adventurers or traders. [...]

 This makes assumptions about the nature of the world and the cultures involved. IIRC Eberron has enough human-elves that they're considered a separate species. In any sufficiently cosmopolitan setting (which ought to include the Forgotten Realms, or at least the most-portrayed parts), they'll probably be not particularly unusual.

Also, assuming PCs are more or less a representative sample of the population, they ought to be incredibly common in most D&D worlds. :)

 
Even in a cosmopolitan setting, it doesn’t necessarily follow that social/cultural barriers won’t be a factor. It’s not exactly unheard of for some demographics to maintain fairly insular communities within the larger community of a great city. That’s a common point of characterization for elves in the FR novels I’ve read, to address your own example.

Quote from The_Ace_of_Rogues >>

 Ideally the upcoming DMG will have the section on “any pairing can have kids with a DM thumbs up, average the lifespan, pick one block from the parents for traits, and appearance is whatever the player and DM agree on”; it’s much better at accommodating the breadth of possibilities people come up with rather than only supporting a tiny percentage of them. 

 Your suggestion would be a very good solution, if the DMG is too far along then a blending of races section in the Monster Manual may have the time to have this added. Some guidance of game mechanics, life span, etc