Muysken (2019) has argued that the most convincing cases of contact-induced change in heritage languages involve the dominant language having two distinctions mapping on to one (2-to-1). Evidence of such a case from Toronto heritage Cantonese will be discussed. Toronto English (the dominant language) has an allophonic split in which the TRAP vowel is raised and fronted in pre-nasal contexts. This is argued to influence the development of a similar allophonic split, led by lower proficiency speakers, in which Cantonese /ɛ/ is fronted before nasal consonants. The lack of an /ɛ/ split in Hong Kong Cantonese provides further support for contact-induced change. Unlike cases of a 1-to-2 mapping leading to a loss of a distinction in the heritage language (which can be argued to be internally motivated), this contact-induced split leads to increased phonological complexity, which is inconsistent with a deficit view of heritage language speech production.
Tse, Holman, "The pre-nasal allophonic splitting of /ɛ/ in Toronto Heritage Cantonese" (2021). English Faculty Scholarship. 89.