Elizabeth Zsiga, Maria Gouskova and One Tlale. 2006. On the status of voiced obstruents in Tswana: Against *ND. . In C. Davis, A. Deal, and Y. Zabbal (eds.) The Proceedings of NELS 36. Amherst, MA: GLSA. pp. 721-734.
Tswana is sometimes presented as a case of an “unnatural” phonological process that goes counter to the phonetically grounded constraint against voiceless stops after nasals (*NC). The language reportedly bans not voiceless but voiced stops after nasals ([bata] ‘look for,’ [mpata] ‘look for me’). A closer look at the phonology of Tswana and a phonetic study suggests that this description is not accurate. For some speakers, no stops are ever voiced, including post-nasal stops. For others, the consonants are voiceless not only after nasals but also between vowels. The study shows that Tswana provides no evidence against *NC.