Languages are arbitrary. The word for “dog” in English sounds nothing like the word for “dog” in Mandarin, because the connection between sound and meaning is conventional, not natural. This is one of the foundational principles of modern linguistics, established by Saussure and confirmed across a century of cross-linguistic research. Arbitrary mapping is considered the default.
Pain breaks the rule. Ponsonnet, Pisanski, and Coupé analyzed over 600 pain interjections across 131 languages and found that pain expressions converge on the same vowel: open “a,” the sound in “ah.” Pain interjections also feature wide falling diphthongs — “ai” as in the English “ay,” “au” as in “ow.” Joy interjections and disgust interjections showed no such convergence. They vary freely across languages, following the expected arbitrary pattern. Only pain sounds the same everywhere.
The convergence tracks the biology. Nonlinguistic pain cries — the involuntary vocalizations that precede and accompany language — share the same open-vowel structure. An open “a” is the sound produced when the vocal tract is maximally open, which is the configuration the mouth assumes during an involuntary cry. Pain interjections in language didn't inherit their sound from a cultural convention. They inherited it from the cry itself — the reflexive vocalization that language later formalized into a word.
The arbitrary system has a seam. Where language maps meaning to sound, the mapping is conventional and varies freely. But where language maps sensation to sound — specifically, where the sensation is intense enough to produce an involuntary vocalization — the mapping is constrained by the body. Pain interjections are not words that happen to sound alike. They are cries that happen to have become words.