6-month-olds are sensitive to English morphology

Each language has its unique way to mark grammatical information such as gender, number and tense. For example, English marks number and tense/aspect information with morphological suffixes (e.g., -s or -ed). These morphological suffixes are crucial for language acquisition as they are the basic building blocks of syntax, encode relationships, and convey meaning. Previous research shows that English-learning infants recognize morphological suffixes attached to nonce words by the end of the first year, although even 8-month-olds recognize them when they are attached to known words. These results support an acquisition trajectory where discovery of meaning guides infants' acquisition of morphological suffixes. In this paper, we re-evaluated English-learning infants' knowledge of morphological suffixes in the first year of life. We found that 6-month-olds successfully segmented nonce words suffixed with -s, -ing, -ed and a pseudo-morpheme -sh. Additionally, they related nonce words suffixed with -s, but not -ing, -ed or a pseudo-morpheme -sh and stems. By 8-months, infants were also able to relate nonce words suffixed with -ing and stems. Our results show that infants demonstrate knowledge of morphological relatedness from the earliest stages of acquisition. They do so even in the absence of access to meaning. Based on these results, we argue for a developmental timeline where the acquisition of morphology is, at least, concurrent with the acquisition of phonology and meaning.