Dissimilation


 * For the chemical term, see catabolism

In phonology, particularly within historical linguistics, dissimilation is a phenomenon whereby similar consonant or vowel sounds in a word become less similar. For example, when one sound occurs before another in the middle of a word in rhotic dialects of English, the first tends to drop out, as in "beserk" for berserk, "supprise" for surprise, "paticular" for particular, and "govenor" for governor (this does not affect the pronunciation of government, which has only one ). 

Causes
There are several hypotheses as to what causes dissimilation. John Ohala posits that listeners are confused by sounds that have long-distance acoustic effects. In the case of English, rhoticization spreads across much of the word (that is, in rapid speech many of the vowels may sound like they have an R in them), and it may be difficult to tell whether a word has one source of rhoticity or two. When there are two, a listener might wrongly interpret one as an acoustic effect of the other, and so mentally filter it out.

This factoring out of coarticulatory effects has been experimentally replicated. For example, Greek pakhu- (παχυ-) "thick" derives from an earlier *phakhu-. When test subjects are asked to say the *phakhu- form in casual speech, the aspiration from both consonants pervades both syllables, making the vowels breathy. Listeners hear a single effect—breathy voiced vowels—and attribute it to one rather than both of the consonants, assuming the breathiness on the other syllable to be a long-distance coarticulatory effect, thus replicating the historical change in the Greek word.

If Ohala is correct, one might expect to find dissimilation in other languages with other sounds that frequently cause long-distance effects, such as nasalization and pharyngealization.

Examples
In English r deletion above, when a syllable is unstressed, it may drop out altogether, as in "deteriate" for deteriorate, "tempature" for temperature, and "apeture" for aperture, a process called haplology. When the is found as, it may change to : "Febyuary" for February, "defibyulator" for defibrillator, though this may be due to analogy with words such as January.

Types of dissimilation
Dissimilation, like assimilation, may involve a change in pronunciation relative to a segment that is adjacent to the affected segment or at a distance, and may involve a change relative to a preceding or a following segment. As with assimilation, anticipatory dissimilation is much more common than lag dissimilation, but unlike assimilation, most dissimilation is triggered by non-contiguous segments. Also, while many kinds of assimilation have the character of a sound law, few dissimilations do; most are in the nature of accidents that befall a particular lexical item.

Anticipatory dissimilation at a distance (by far the most common):
 * Latin *medio-diēs ("mid-day", i.e. "noon"; also "south") became merīdiēs. Latin venēnum "poison" > Italian veleno. This category includes a rare example of a systematic sound law, the dissimilation of aspirates in Greek and Sanskrit known as Grassmann's Law: *thi-thē-mi "I put" (with a reduplicated prefix) > Greek tí-thē-mi (τίθημι), *phakhu "thick" > Greek pákhus (πάχυς), *sekhō "I have" > *hekhō > Greek ékhō (ἔχω; cf. future *hekh-s-ō > héksō ἕξω). Some apparent cases are problematic, as in English "eksetera" for etcetera, which may rather be contamination from the numerous forms in eks- (or a combination of influences), though the common misspelling ect. implies dissimilation.

Anticipatory dissimilation from a contiguous segment (very rare):
 * The change from fricative to stop articulation in a sequence of fricatives may belong here: German sechs /zeks/ (as evidenced by the spelling, the was previously a fricative). In Sanskrit in any original sequence of two sibilants the first became a stop (often with further developments): root vas- "dress", fut. vas-sya- > vatsya-; *wiś-s "clan" (nom.sg.) > *viťś > *viṭṣ > viṭ (final clusters are simplified); *wiś-su locative pl. > *viṭṣu > vikṣu. English amphitheater is very commonly pronounced ampitheater (though spelling pronunciation may be either some or all of the story here).

Lag dissimilation at a distance (fairly common):
 * Latin rārus "rare" > Italian rado. Cardamom the spice commonly cardamon. In Middle English, in a whole list of words ending in -n but preceded by an apical consonant the -n changed to -m: seldom, whilom, random, venom. Eng. marble is ultimately from Latin marmor. Russian февраль  "February" is from Lat. Februārius.

Lag dissimilation from a contiguous segment (very rare):
 * Latin hominem ("man", acc.) > Old Spanish omne > omre > Spanish hombre
 * Latin nominem ("name", acc.) > nomre > Sp. nombre
 * English chimney (standard) > chim(b)ley (dialectal)
 * Proto-Slavic *"sveboda" "freedom" > Slovak "sloboda"

Paradigmatic dissimilation
When, through sound change, elements of a grammatical paradigm start to conflate in a way that is not easily remedied through re-wording, the forms may dissimilate. For example, in modern Korean the vowels and  are merging for many people in the capital Seoul, and concurrently the second-person pronoun 네  'you' is shifting to 니  to avoid confusion with the first-person pronoun 내  'me'.

Similarly, it appears that English she, historically heo, may have acquired its modern sh form through dissimilation from he, though it is not clear whether the mechanism was idiosyncratic sound change (palatalization) of heo, or substitution of heo with the feminine demonstrative pronoun seo.