In historical linguistics, lexical diffusion is both a phenomenon and a theory. The phenomenon is that by which a phoneme is modified in a subset of the lexicon, and spreads gradually to other lexical items. For example, in English, /uː/ has changed to /ʊ/ in good and hood but not in food; some dialects have it in hoof and/or roof but others do not; in flood and blood it happened early enough that the words were affected by the change of /ʊ/ to /ʌ/, which is now no longer productive.
The related theory, proposed by William Wang in 1969, is that all sound changes originate in a single word or a small group of words and then spread to other words with a similar phonological make-up, but may not spread to all words in which they potentially could apply. The theory of lexical diffusion stands in contrast to the Neogrammarian hypothesis that a given sound change applies simultaneously to all words in which its context is found.
Mainstream historical linguists reject Wang's hypothesis, continuing to adhere to Neogrammarian exceptionlessness. For example, Pulleyblank regards the theoretical formulation of lexical diffusion as presented by Hsieh in Wang 1977 as "so manifestly at odds with any realistic picture of how dialects are inter-related and how innovations spread spatially through a language as to make them totally untenable" (1982: 408).
Referring to one of Wang's touchstones of lexical diffusion, Egerod dismisses his theory as a sleight of hand:
there is no "massive split" involved, but an error of methodology in accounting for tones. Cháozhōu like other languages in China or outside of China has a complicated history with migration waves, loans and analogical formation. The conscientious historical linguist has to account for these before he resorts to a deus ex machina (1981: 173).
Mazaudon & Lowe conclude a robust critique of lexical diffusion in a similar vein, remarking that "a detailed study of the history of the language can disentangle the reflexes from different sources, and it is not necessary to renounce the principle of regular change for the sake of such cases" (1993: 11).
William Labov, in Principles of Linguistic Change, takes the position that there are two types of sound changes: regular sound change (respecting the Neogrammarian hypothesis) and lexical diffusion. Labov lists a typology, according to which certain phenomena are typically or exclusively regular (example, vowel quality changes), while others (example, metathesis, or vowel shortening) tend to follow a lexical diffusion pattern.
Paul Kiparsky, in the Handbook of Phonology (Goldsmith editor), argues that under a proper definition of analogy as optimization, lexical diffusion is not a type of sound change. Instead, Kiparsky claims it is similar to leveling, in that it is a non-proportional type of analogy.
- Wang, William S-Y. (1969). "Competing Changes as a Cause of Residue". Language. 45 (1): 9–25. JSTOR 411748.
- Pulleyblank, Edwin G. (1982). "The Lexicon in Phonological Change. Monographs on Linguistic Analysis, no. 5 by William S-Y Wang". Journal of Chinese Linguistics. 10 (2): 392–416. JSTOR 23767018.
- Egerod, Søren (1982). "How not to split tones—the Chaozhou case". Fangyan. 3: 169–173.
- Mazaudon, Martine; Lowe, John B. (1993). "Regularity and Exceptions in Sound Change". In Domenici, Marc; Demolin, Didier (eds.). Annual Conference of the Linguistic Society of Belgium. Brussels. pp. 1–25.
- Labov, William (1994). Principles of Linguistic Change, Volume 1: Internal Factors. Cambridge, Massachusetts: Blackwell. pp. 421–543. ISBN 978-0-631-17913-9.
- Kiparsky, Paul (1995). "The phonological basis of sound change". In John A. Goldsmith (ed.). The Handbook of Phonological Theory. Cambridge, Massachusetts: Blackwell. pp. 640–670. ISBN 0-631-18062-1.
- Phillips, Betty (2006). Word Frequency and Lexical Diffusion. New York: Palgrave MacMillan. ISBN 978-1-4039-3232-7.
- Phillips, Betty S. (2015). "Lexical Diffusion in Historical Phonology". In Honeybone, Patrick; Salmons, Joseph (eds.). The Oxford Handbook of Historical Phonology. pp. 359–373. ISBN 978-0-19-923281-9.
- Wang, William S.-Y., ed. (1977). The Lexicon in Phonological Change. Monographs on Linguistic Analysis. 5. Mouton De Gruyter. ISBN 978-3-11-177423-7.
- Hashimoto, Mantaro J. (1981). "The lexicon in phonological change Edited by William S-Y. Wang (review)". Language. 57 (1): 183–191. doi:10.1353/lan.1981.0053. JSTOR 414291.
- Walker, Douglas C. (1979). "The lexicon in phonological change: W.S.Y. Wang, Mouton, The Hague, 1977 Monographs on Linguistic Analysis 5. 278 pp. 112 DM". Lingua. 49 (4): 361–363. doi:10.1016/0024-3841(79)90050-0.