Nonverbal predication and SELF-assertive truth-conditionality in American Sign Language

Our understanding of nonverbal predication wherein canonically non-verbal phrases such as nouns and adjectives are predicated in American Sign Language (ASL) is fragmentary. ASL has been reported to make extensive use of the null copula in nominal predication, but this claim was countered by recent evidence of an overt copula in the form of the sign SELF. There is also scant description and evidence regarding the structure of adjectival predication in ASL. Thus, we undertake two Likert-scale experiments investigating the relationship between several signs serving as potential copulas (SELF, IX, and null) and a variety of non-verbal predicates including nouns and adjectives in ASL. We find that SELF appears to be the primary means of expressing nominal predication as a copula in contrast to IX and null. As for adjectival predication, there is a preference to use the copular SELF with adjectives exhibiting more permanent properties such as DEAF and TALL. Paired with the finding that all adjectives in ASL appear to be predicative, we propose that the copular SELF carries an additional semantic contribution of grammatical modality. Signers appear to use it to assert truth and certainty, which is a novel finding in sign languages. We conclude that this grammaticalization process in sign languages parallels the account that lexical information historically expressed using non-manual markers eventually takes on a manual form.