Every child learns their first words at a different pace. Research on first language acquisition has studied the individual variation of children’s linguistic development for decades, trying to determine the contributing factors for this variation [28, 29, 23].
Ever since the seminal study by Hart and Risley , the amount of caregiver input has been in the focus of attention as a main variable relevant for early vocabulary acquisition. Hart and Risley attributed individual differences in young children’s language acquisition to the amount of parental talk they heard. They observed that socioeconomic status (SES) correlated with the quantity (eg word count) and the quality (eg utterance complexity and lexical diversity) of speech addressed to children. At the time, the general consensus seemed to be that caregivers should provide as much verbal input to infants as possible to give them a head-start in language learning.