What could prompt an elvish society to develop monogamy?

[Rosemary Tea]

Perhaps but there are many variables to take into account when determining what the individual composition of four parent families might be and how many children there might be within such a household. For example, families could be made up of FFFM, FFMM or FMMM if they wish to reproduce biologically. My scenario also did not take into account the availability of contraception of some sort which would also influence the make-up of a household nor those who are not biologically male or female.

The OP specified that the four parent families in their scenario are all MMFF. So their reproductive capabilities would be exactly the same as two monogamous MF couples.

How many children they would choose to have, assuming they can control their reproduction enough to make that choice, might correlate with how many parents there are. Perhaps two parent families would end up having more children because they keep having boys and they keep trying again for a girl, or vice versa, whereas in a four parent family, if one mother gives birth to nothing but boys and the other gives birth to nothing but girls, they still get to parent both. Or perhaps they'd have fewer because they want to keep the number of children down so inheritance doesn't get spread too thin. Or more because they're farmers and they need all those children to work the land.

In all of those cases, a couple would be likely to have more children than they would if they were part of a foursome.

 

[Qvadrater]

One explanation could be space constraints: there simply aren't a lot of large enough contiguous living spaces to house such large families, forcing families to become smaller, and therefore splitting into monogamous pairs requiring roughly half the contiguous space per family.

This could also lead to some interesting familial and social diversity: if there are just a few living spaces large enough for tesseramory families these would be more expensive and only available to rich upper-class elves.

I've no idea what would prompt the space constraints though ¯\_(ツ)_/¯.

 

[WooHooMan]

Even among non-heterosexual people monogamy is almost universally seen as the ideal relationship in the West but this isn't necessarily the case in non-Western societies.

Nearly every society, including the west, had accepted polygamy at some point. And in addition to the west, polygamy isn’t exactly common in modern east Asia. Then there’s the tolerance for non-heterosexuality outside the west (or past west vs. modern west) which, y’know, is its own topic.

So assuming elves are somewhat similar to humans, I’d think it’s very possible for them to switch to monogamy even if it wasn’t considered their “ideal” or norm beforehand. The question is really, why would an elf settle for one partner when they could have three?