Historical transfer of nasality between consonantal onset and vowel

Comparative data from several language families show that nasality can be transferred between a syllable-initial consonant cluster and the following vowel. The cases reported to date are summarized, and a new analysis is proposed for a set of Sino-Tibetan data. The evolution appears to go in both directions: from the consonantal onset to the following vowel in Tai-Kadai, Austroasiatic, Sino-Tibetan, Niger-Congo (Kwa) and Indo-European (Celtic), and from the vowel to the preceding consonant in Siouan. However, an examination of the conditions on these changes brings out an asymmetry. In most cases, transfers of nasality take place from a consonantal onset to a following vowel; the instances we found of a regular change in the opposite direction all come from languages where there is one of the following restrictions on nasal sounds: (i) nasal consonants are nonphonemic (contextually predictable), or (ii) the opposition between nasal and oral vowels is neutralized after nasal consonants (in favor of nasal vowels).