Child acquisition of syntactic and phonological variation

While categorical variation exists in both syntax (e.g., EModE do-support vs. V-to-I) and phonology (e.g., English /r/-deletion), phonological variation also encompasses subcategorical phonetic adjustments, with many categories occupying a continuous range of values (e.g., English /u/-fronting). Moreover, many phonological patterns are grounded in physiological properties, e.g., /ki/ → [tʃi] alternation, which is motivated by articulatory, acoustic, and perceptual factors (Keating & Lahiri, 1993; Guion, 1998; Butcher & Tabain, 2004) that enhance its learnability (Wilson, 2006). Thus, while learning in both domains is subject to cognitive biases, phonological learning may also be subject to bias from physical factors.

Several studies have compared acquisition in the two domains through artificial language learning experiments. However, the tasks often differ in complexity (Ferdinand et al., 2019; Hudson Kam & Chang, 2009) or test different populations, e.g., English-speaking (Hudson Kam & Newport, 2009) vs. Cantonese-speaking childen (Do & Mooney 2021). We examine whether morphosyntactic and phonological learning differ when children learn artificial languages of equal complexity.

Seventy-six 5–6-year-old Cantonese-speaking children learned an artificial language involving one of (a) categorical gender agreement, (b) variable gender agreement, (c) categorical rounding harmony, or (d) variable rounding harmony. All languages were composed of 4 CVCV kinship terms (2 masculine, 2 feminine), e.g., [miko] ‘brother’, [pita] ‘sister’. Each noun was presented with 6 CV adjectives denoting an emotion, e.g., [le] ‘happy’, totaling 24 training items, e.g., [le miko] ‘happy brother’. Each item was visually accompanied by a corresponding cartoon depiction. In syntactic learning, the adjective showed feminine and masculine allomorphs, e.g., lo ~ le. In phonological learning, the adjective contained unround [e] or round [o], harmonizing with the first vowel of the following noun, [le miko] vs. [lo nuto]. In variable conditions (b) and (d), agreement appeared in 67% of training items.

Children were tested on generalization to 24 novel nouns. In categorical tasks, participants showed comparable rates of harmony/agreement (83.9% syntax; 83.4% phonology). In variable learning, the syntactic and phonological learning tasks diverge. In the variable phonological task, the application of harmony exceeded the rate of exposure in training (74% with 67% exposure), suggesting the influence of a naturalness bias whereby children prefer phonetically natural rounding harmony to phonetically unnatural disharmony. In the variable syntactic task, participants applied gender agreement below the rate of exposure (63% with 67% exposure). This finding points to a qualitative difference between learning in the two domains, with phonological learning influenced by substantive grounding.