...I don't think so?

I can't think of any. Or - not ones that we've since figured out. The rules about marriage are a thing which they said but didn't explain, but we haven't really had enough examples of people doing the thing they didn't recommend to know why they said it.

"'Cause, like, humans could tell you about ecology too, at least simple things like overhunting, but we obviously don't have politics sorted out."

There are probably also examples of them knowing things humans haven't figured out, but unless we have independently figured out the reason for their rule I wouldn't be able to explain it to you except as "there's a rule".

"What've you got?"

Uh, there are places where no one should walk more than once a century, no one should work while their children are not fully grown unless it's work that makes them happy and refreshed to appreciate the children more, you shouldn't pick your children's sex - my parents did that and it does not seem to have gone horribly but I guess if everyone did, then it might? - you shouldn't try to go to Endorë, you shouldn't do things that kill you, you shouldn't get married before you're fifty, you shouldn't do intimate things which don't marry you outside of marriage, you shouldn't have a baby the first year you're married, you shouldn't pursue someone you can't marry, you shouldn't speak badly of Eru...

"Is once a century an exact figure or is it just a way of saying keep it to a minimum? That could be ecological too, fragile slow-growing things."

I think it's about keeping it to a minimum, but I'm not sure.

There's 'you shouldn't go around with your hair loose', 'you shouldn't disobey the King'...

"...the hair thing I have no idea -" She brings her hand up to the nape of her neck.

Elves look away! It's just - you don't do that. 

"- do you know why, or is this just a Vala edict that stuck real hard?"

I can't really imagine that people just touched their hair in public before the Valar said that hair is private. I think they might have had different rules, like, some places you covered with fabric and some just tying it was fine, but I'm sure everywhere had a rule, how could you not? 

"We don't, but maybe that's biological somehow."

It seems likely.

"The sex selection one happens in a couple places on Earth, and I think it causes some statistical-level friction when one sex is much less popular - girls, anyway, maybe things would be okay if more people wanted girls but in practice it's boys they want. I'd expect it to be less of a problem for an immortal species, though, the issue is, like, human men of a certain age range are likelier to cause trouble and having a girlfriend or wife will distract them or something? Since most people are heterosexual? I haven't looked deeply into this I'm speculating based on two headlines and a paragraph and the campaign copy about not buying fertility drugs."

Our father wanted boys because probably girls cannot inherit and he did not want his half-siblings to inherit but this is an unusual reason and I don't think most peoples' picking would be skewed one way or the other? I suppose they might have more girls if they wanted them to marry into the royal family. Men who are unmarried do not need a wife to distract them from causing trouble but I suppose people might be sad to be long unmarried for lack of possible partners.

"Yes, being sad about it seems like the gentle Valinor version."

So maybe that's the reason for that one. Or that it will cause people to have strange ideas about genders if they're unbalanced, that could also be it. 

"And most of the rest of it sounds more... or less... benevolently paternalist."

None of the rules have ever seemed very unreasonable to me but it would be nice if they were better at explaining reasons.

"Bunch of them would not work so well with humans but we are as established different species."

Yes. And hopefully the Valar would have different rules if they were hosting humans.

"Hopefully, but some of the rules sound... a lot like some historical or even currently implemented human rules? Which were very costly in terms of well-being for a lot of people and evolved for reasons like being sure of paternity, that I'm gonna guess you don't have, so... maybe there are other reasons but..."

Which of those rules were costly for humans?