Abstract: Morphological complexity is expected to decrease under mass admixture from adult second language speakers. While this has been chiefly shown for morphological richness, an unresolved question is whether the effect extends to aspects of morphological boundedness. Here we report a case study of Sino-Tibetan verbs, contrasting verbal expressions of two languages with very large (Chinese, Burmese) and of two languages with very small (Bunan, Chintang) numbers of second language speakers. We find that while the amount of second language speakers accounts for differences in the range and number of inflectional categories (degrees of synthesis), it does not affect the way in which morphological constituents are bound together, reflecting fortification through a mix of diachronically stable and universally preferred patterns. This calls for theoretical models that narrow down the range of changes that are driven by second language speaker admixture, and for extensive empirical testing on a global scale.