Why do characters with a high prime requisite gain bonus XP?

Question

In earlier versions of Dungeons & Dragons, a character with a high ability score in their class' prime requisite receives a bonus to all experience he earns. For example, the 1983 Frank Mentzer edition included this chart:

Prime Requisite Score  Adjustment to Experience
        3-5                     -20%
        6-8                     -10%
        9-12                No adjustment
       13-15                    + 5%
       16-18                    +10%

What is the reason for this rule? Did the game's designers ever explain it, either officially or anecdotally?

Answer 1

When building characters, with "in order" statistic generation, or "assign existing roll" statistic generation, the XP bonus acts as an incentive to produce two outcomes. For "in order" character generation, it strongly suggests to players that they make their character "fit" their statistics, in particular by aligning their character selection with their highest statistic. No INT18 fighters please, we're Gygaxian. Correspondingly, with player assigned statistics (and points buy, obviously), it encourages characters whose statistics match their function in at least one statistic, their primary one. No STR18 WIS9 clerics please, we're Gygaxian.

Given that primary statistics also limit spells and levels; there is a strong rules based encouragement for character statistics to match assumed "rule ideal" types of characters.

---

Secondly, with statistic degrading poisons, traps, effects and magic; the loss of XP when your primary statistic is degraded below 9, is an encouragement for Fighters to regain their Strength, Clerics to regain their Wisdom, etc. Again, this encourages actual characters to match a rule ideal "type" of character at the level of statistics.

Answer 2

This is just an opinion, but seeing the bonuses in original D&D (OD&D) since the beginning, they seem to be a way to subtly push the player towards a specific character class. Remember, in the early days, many systems (including OD&D) required you to roll three dice, in order, for your stats. So many players who wanted to work a fighter might end up with a 9 strength and a 17 wisdom, and they would just play the fighter despite having bonuses should they have played the cleric. It appears the prime requisite bonuses might be there to reward a player for "taking a chance" on a character class he might not normally choose. The 5-10 percent added to experience (in contrast to the 10-20 percent LOST if the prime requisite was 8 or under) would be a substantial bonus in early editions of the game where EXP were sometimes scarce and character longevity was not guaranteed.

EDIT: BTW, per a question by Yogo, here is the original Prime Requisite bonuses from the OD&D rules. It is interesting to note the changes from this to later editions:

In OD&D there are only three classes: Fighting Men, Magic Users, Clerics. All abilities are rolled by 3d6, in order.

STR is prime requisite for Fighting-Men. INT is prime requisite for Magic Users. WIS is the prime requisite for Clerics.

Bonuses and Penalties for Advancement Due to Abilities: Low score is under 8; Average is 9-12; High is over 13

Prime Requisite 15 or more: Add 10% to earned Experience Prime Requisite 13 or 14: Add 5% to earned Experience Prime Requisite 7 or 8: Minus 10% from earned Experience Prime Requisite 6 or less: Minus 20% from earned experience

There are ability bonuses given for high CON, DEX or CHR, but they are unrelated to Experience point awards. Interestingly, besides the Experience point bonuses there is little advantage to having a high STR, INT or WIS except for intangibles: A high STR is said to help in opening traps, A high INT will add languages and affect referees decisions about certain actions the player might make, A high WIS acts in the same way as a high INT (might affect decisions made per the referee's decision)

Answer 3

Justin Alexander had an interesting discussion of how prime requisites and other odd aspects of old-school play worked here. Doesn't quite answer the question (I think those above me have done so quite well), but it is worth reading if you're curious about how the whole construct worked.

Answer 4

I'm only guessing here, but for the majority of classes under those systems, you were required to have a high value in your prime requisite, but there was virtually no system bonus for those high values. Sure, Str, Dex and Con gave all characters in-game bonuses, but high Int gave you nothing but languages. So, to me, the extra XP was a way of giving those high stats an in-game bonus.