Palatalization is a well-known and widespread phonological phenomenon, and is most commonly caused by front vowels or glides. In Setswana, a much different pattern of palatalization occurs: alveolar stridents /s ts ts?/ become pre-palatal [? t? t??] before back vowels and [w].

   1.  Setswana S → Š before [?]
   a.   -hisa ‘burn’         s?-hi?? ‘burner’
   b.   -?misa ‘dry’        s?-womi?? ‘dryer’
   c.   -b?ntsa ‘show’    p?nt?? ‘showing’

Palatalization before back vowels seems phonetically UNnatural, and counter-exemplifies established cross-linguistic norms – but its existence in Setswana is nonetheless quite robust. How can this be? This talk proposes that the historical origin of the Setswana pattern lies in the roundness of back vowels. Lip rounding in post-alveolar stridents is a means of phonetically enhancing the S~Š distinction (Keyser & Stevens 2006). As such, if rounding of back vowels spread onto preceding sibilants, it would cause [s ts ts?] to be perceived as more similar to [? t? t??] – leading to palatalization only before back vowels and [w].