Kerinci (a group of related Malayic isolects) developed agreement-like registration of nominal complements—realized phonologically as ablaut: for example, bantɨ 'pillow' (basic form) versus bantʌn (secondary form). As we showed in earlier work, this new morphology led to radical changes in Kerinci grammar: for example, the occurrence of morphologically licensed null pronouns, which allow apparent extraction from island environments. What has, until now, remained mysterious is how Kerinci became so different from other Malayic languages.
Until recently, it was believed that Kerinci was unique in manifesting stem ablaut. We show that phonological alternations similar to those in Kerinci exist in previously undescribed non-Kerinci Sumatran varieties, yet these alternations are not accompanied by the radical grammatical changes seen in Kerinci. By comparing Kerinci and non-Kerinci varieties, we show that parallel phonological changes driven by structural pressures shared by Malayic languages contributed to ablaut developing independently in several regions. In Kerinci, however, phonological opacity, due to overlapping environments in two previously distinct phonological alternations, led to the reanalysis of the environment for ablaut as morphosyntactic, thereby resulting in a major structural shift.