Interface vulnerability in bilingual grammars? Spanish production of null and overt subjects in Basque-Spanish bilingual children and adults

Bilingual children typically produce more redundant pronominal subjects than monolingual children in null subject languages like Spanish in contact with non-null subject languages like English (Montrul & S & aacute;nchez-Walker, 2013; Montrul, 2004; Paradis & Navarro, 2003; Silva-Corval & aacute;n, 1994). According to the Interface Hypothesis (Sorace and Filiaci, 2006; Sorace and Serratrice, 2009; Sorace et. al. 2009; Sorace, 2011), the overextension of overt pronouns among bilinguals stems from interface constraints and processing limitations. Recent research with adults (Giannakou, 2018) found that the predictions of the Interface Hypothesis were not supported by bilingual performance in contact situations between two null subject languages, suggesting that language transfer may be at play. This study investigates the acquisition and development of pronominal subject expression in 196 school-age Spanish monolingual and Basque-Spanish bilingual children (ages 6-12), as well as adults, through a pronoun elicitation task. Basque and Spanish are both null subject languages. Findings suggest that monolingual Spanish speakers produced more redundant overt pronouns than the bilinguals, and there was overextension of null pronouns in switch-reference contexts in both populations. Not only do these results disconfirm the Interface Hypothesis, but they also support the conclusion that null and overt pronouns display a variable distribution in null subject languages.