Just read a piece describing ablaut reduplication, where the first vowel is almost always a high vowel and the reduplicated ablaut variant of the vowel is a low vowel.
Examples: chit-chat, ding-dong, knick-knack, flip-flop, zig-zag, flimflam, hip hop
I'm curious about the underlying mechanism that gives rise to this pattern... Is it that high-to-low vowel is just easier for the muscles involved in speech? Is there other physiology or psychology involved?
This is a version of the general phenomenon of fixed-segmental reduplication. Many languages have partial reduplication where a particular element is a fixed phoneme (e.g. di-dō from /dō/; gbi-gbona). A number (smaller) have complete reduplication: rimisirana-rimisirana. The English echo reduplications mentioned above combine those two properties; likewise, Kolami maasur-giisur ('men and the like') has a fixed constituent (the onset of the second syllable). Replacive vowel patterns are also found in a number of languages, such as the Munda language Gta', where root vowels are replaced in various ways for various effects (e.g. "gross", "tender", "different"), so fixed segmentalism is not limited to reduplication. (I would be remiss if I ignored Semitic root-and-pattern changes in vocalism).
The ultimate historical explanation for the specific vowel pattern found in English is not clear, but it is something that languages do. It is fairly common in "affective" word-formation (i.e. processes that convey an added nuance to a meaning, as opposed to being a grammatical inflection). There does not seem to be any extensive study that assembles the facts and tries to discern universal patterns (e.g. is [ɪ-æ] more common than [æ-ɪ]). If the English choices and orders of vowels is just one of a myriad set of attested patterns, then there isn't much to explain other than "what happened that this pattern got fixed and not [ɛ-u]?". It's really not know if there are significantly recurring patterns across languages.
I found one study*, and though far too detailed and technical in its body for my purposes, seems to support the author's initial hypothesis, that 'basically, it just sounds good' [my paraphrase]. :/ I suppose if I wanted to pursue further, I would look for psycho- and/or physio-linguistic experts/reports.
Excerpt from Summary: "The question of why English Ablaut reduplication imposes specific restrictions on the choice of vowels in the two halves of the structure has been addressed before, but the answers have not covered the empirical facts in a satisfactory way. This study proceeds from the assumption that English Ablaut reduplication follows a well-defined dipodic template composed of syllabic trochees. The structure of the template is justified on the basis of independently available prosodic and segmental characteristics of the parts and the whole of the string. The two halves of the structure are treated as morphological equals and their relationship is evaluated in terms of output-to-output copying. A product of its time, the account uses Optimality Theory, positing constraints on identity in reduplication and general segmental and prosodic markedness and well-formedness constraints. The latter jointly satisfy a principle valid for other verbal art patterns relying on the iteration of linguistic sames, the principle of Interest. The Ablaut template enriches the taxonomy of verbal art structures in English."
