[RPG] When an enethe is defeated, who in the party gains experience?

The ultimate and most ancient point system for "quantizing success through a numerical method" is, of course, money. Or perhaps predating even that, number of cattle, sheep, size of land controlled, etc. And war and trade were very early human activities to optimize that quantized success.

History aside, LordVreeg's answer above looks the most promising to me so far in terms of modern gaming.

Update

Since my original post, I have found a online-viewable citation that clearly credits David Arneson (co-creator of D&D with Gary Gygax) as creator of the "experience (point) system". It comes from a March 2008 Wired Magazine article, "Dungeon Master: The Life and Legacy of Gary Gygax" by David Kushner, and was written shortly after Gygax died (2008). It describes the creation/development/motivation of the experience point system this way:

Arneson tested his Chainmail mod in play sessions with his group and, based on their feedback, continued to tinker with the rules to make it more fun. ...

There was another aspect of the game he wanted to tweak: the fact that it ended. Arneson's group was having too much fun playing these specific roles to want to part with them after a single game. Outside of the individual games, Arneson created an experience system for characters. Your character would earn experience points based on their success from game to game. After a certain number of poins [sic], a character would "level up."

It does seem to be clear from this point that experience points were not in Chainmail and were Arneson's invention and part of his contribution to D&D. It does not necessarily rule in or out whether Areneson pulled the idea from somewhere else however.

Your question and other comments do leave me curious to know about ancient competitions (e.g. gladiators, tournaments such as jousting tournaments)-- whether they had any cumulative or longer-term rankings (based on win/loss ratios or #wins) that would be quantitative and cumulative in nature and thus similar to XP beyond what you'd just find in "sports".

I think it's a great topic worthy of intellectual and historical research. What you call "currency of achievement" is what I think of as a "Cumulative point system". As I've reflected upon my own behavior and others' over time, and beyond just the sphere of "games", I've found that cumulative point systems-- whether found in games, online forum reputation systems, or real-world systems like money -- tend to have three great virtues that make them addictive-behavior generating:

• immediate feedback (delayed feedback reduces addictiveness)
• clearly recognizable feedback (quantization adds clarity that increases addictiveness)
• effort compounds over time (benefits that last into the indefinite future are much more valuable than transient ones, increasing addictiveness)

Good luck with your research!

As you say, this house rule works well for your group. A new player objecting to a house rule they don't understand is no reason for you to change it.

And they don't understand the point of the house rule. They've observed their fellow players and seen the rule's results, and are trying to adjust their actions to fit what the rule is meant to encourage — and they've misunderstood. Instead of engaging in new, creative roleplaying, they're just copying what they see other players do in a kind of cargo cult roleplaying.

This is also the source of their frustration: they think they've solved the puzzle, and they perceive you denying them the reward for solving it. They haven't understood it though, and rewarding them for missing the point will not help them integrate into your group.

Having put in effort and missed the "target" of understanding the point of the bonus experience, though, you should meet them halfway:

"It's clever how you noticed what the other players did to earn the bonus experience. That's great, but it's only halfway to getting the point of the bonus. The bonus is a reward for individual creativity in roleplaying, so simple copying doesn't earn the bonus.

"Does that make sense? This is an important tool that has worked for us, and I want you to understand what it's for so that it works for you too."

You need to emphasise that you're willing to help the new player get up to speed with the group. Emphasise that, because that's what the conversation should be about — not about whether the rule is useful or not (you already know it is for your group), and not whether the player gets to challenge the award (that's not their job in the group dynamic). Just take it as given that this is how it works, that's not up for discussion and don't bring it up for discussion. Move past those givens without even mentioning them, and instead move right to discussing with the player the point of it and how they can reach that point.

P.S. — Contrary to recent online RPG community fashions that resist XP rewards, this is a common and widespread way for RPGs and groups to work, and entirely functional when done fairly and impartially. Current fashion is simply that — passing fashion. Fashion is not legit grounds to tell you it's a bad way to run your game.