Testing the effects of congruence in adult multilingual acquisition with implications for creole genesis

Linguists from across sub-disciplines have noted that congruence (i.e., form-function mapping) across languages in contact seems to affect acquisition and play a role in language emergence (e.g. Creole genesis). However, because congruence is often confounded with other variables (e.g., frequency, language type, speakers' proficiency levels, perceptual salience, semantic transparency), it remains unclear whether congruence per se benefits learners. In this paper, we provide an experimental test of the effects of congruence on acquisition through an artificial language-learning experiment involving English (L1) and two artificial languages (Flugerdu and Zamperese). English-speakers who identified as "native" (i.e., first-language) speakers (N = 163) were randomly assigned to one of four conditions, varying which of the three languages expressed negation with congruent forms: all three languages; only Flugerdu and Zamperese; only English and Flugerdu; or none. Our findings show that participants better acquired the negation morpheme when the form was congruent with negation in English but not when the two artificial languages alone shared a congruent form. We likewise found unanticipated spillover effects in which participants better acquired the vocabulary and grammar of the artificial languages when all three languages had congruent negation forms. These findings provide insight into the effects of congruence on language acquisition in multilingual environments and Creole language formation.