From the little research that exists on the onset of word learning in infants under the age of 1 year, the evidence suggests an idiosyncratic comprehensive vocabulary is developing. To further this field, we tested 49 nine-month-old infants by pre-assessing their vocabularies using a UK version of the MacArthur-Bates Communicative Developmental Inventory. Intermodal preferential looking (IPL) was then used to examine word comprehension including: (a) words parents reported as understood, (b) words infants are expected to understand according to age-related frequency data, and (c) words parents had reported infants not to understand. Assuming parents are good assessors of their infant's early word knowledge, we expected a naming effect with IPL in condition (a), but not condition (c). As language research uses standard samples of words, we expected a discernible naming effect in condition (b). Results show clear IPL evidence of word comprehension for those words that parents reported their infants to understand (condition a). This agreement between methods demonstrates the usefulness of parental communicative developmental inventory in conjunction with IPL to assess infant's individual word knowledge. No naming effects were found for condition (c) and the lack of naming effects in (b) shows that pre-established word lists may not give a sufficiently clear picture of infant's true vocabulary - an important insight for researchers and practitioners alike. Statement of contribution What is already known on this subject? Most word comprehension research is mainly based on older infants (12, 15, or 18 months of age to 2-3 years and older). Some evidence of word comprehension for common and novel nouns in 6- to 10-month-olds. Existing evidence uses either only specific word groups or nouns combined with specific training and/or repetition procedures. What does this study add? Nine-month-olds display word knowledge independent of context and without repetitions of words. First words encompass not only nouns, but a range of other word classes. Parents are good at indicating which words their infants do and do not understand.
Keywords: comprehension; infancy; preferential looking.
© 2016 The British Psychological Society.