At the recent LIPP Symposium on parts of speech in Munich, one of the most interesting papers was Jennifer L. Smith’s paper on “Parts of speech in phonology” (see also Smith 2011). She examines 20 languages from 11 families and finds that in almost all cases where nouns, verbs and adjectives behave differently phonologically, the nouns have more phonological contrasts than adjectives, and these in turn have more contrasts than verbs. A well-known example is Spanish, where stress is contrastive in nouns (cf. 1a-b) and adjectives, but verbs show uniform stress for all verb classes.
(1) a. sábana ‘sheet’
b. sabána ‘savanna’
Hebrew is similar, but adjectives are intermediate between nouns (with variable stress) and verbs (with uniform stress) in that stress variability is more constrained than in nouns. So Smith formulates a “hierarchy of phonological privilege” N > A > V, and she observes that the word-class effects are overwhelmingly prosodic in nature, involving stress, pitch accent (as in Japanese), tone (as in Ewe), and prosodic shape (as in Chukchi). Another language where the prosodic shape of nouns and verbs is different is Oko (a Benue-Congo language of Nigeria), where verbs must begin with a consonant, and nouns must begin with a vowel, and where even loanwords must obey this, so that English-derived consonant-initial words take an epenthetic initial vowel (e.g. ifiriji ‘fridge’, Atoyebi 2010).
That the phonology can refer to grammatical information also means, according to Smith, that words cannot be category-less in the lexicon, as some recent generative syntactic proposals would have it (e.g. Embick & Marantz 2008): Speakers must store word-class information and phonological information in the same place. (Another very special example of this sort is the mixed language Michif, where nouns are derived from French and verbs are derived from Cree, and both keep the phonological patterns of the source languages.) Smith also notes that the hierarchy N > A > V is more in line with functional-typological clines than with generative binary-feature models. This also fits with the statistical nature of the generalization: There is at least one exception, Ewe, which has more tone distinctions in verbs than in nouns.
What could be the origin of these phonological distinctions? Smith notes that verbs show more inflection that nouns. Using several relevant chapters from WALS, she finds the following:
(2) a. Languages with noun inflection also have verb inflection 94% of the time.
b. Languages with verb inflection also have noun inflection only 76% of the time.
So verbs tend to me more inflected than nouns. (Even if I am skeptical concerning our ability to distinguish inflection from associated clitics, it does seem likely that we would get very similar results if we counted all associated function-items, rather than only inflection.) How can this lead to prosodic differences? Smith speculates about a bias in language acquisition:
“It may be the case that learners have more cognitive resources available to learn complex phonological patterns in nouns than in verbs”
But she does not say why the bias should come from acquisition rather than language use. So this could be a usage-bias affecting all speakers which can get grammaticalized in language change. In any event, it must be a very small bias, because apparently most languages do not show word-class differences in their phonology, and even when they do, the difference does not always mean that one word-class is more privileged than another (i.e. allows more contrasts). But world-wide cross-linguistic work can detect even fairly weak tendencies, and we might eventually find the right explanation.
One might ask further whether phonological differences can also be associated with smaller classes of words, e.g. transitive verbs, or mass nouns, or non-gradable adjectives. Probably examples of all these can be found, they just need to be systematically collected. Another example that Smith mentioned at the conference is proper names, which sometimes show different stress patterns, as is most famously found in Turkish.