Norwegian parents do not modulate infant-directed speech based on their infants' attributed word knowledge

Infant-directed speech (IDS), the register caregivers often use when interacting with infants, has been suggested to facilitate language learning. To assess its didactic role, we examined the acoustic modifications parents make in IDS for words they attribute to their infant as unknown, versus known, by analyzing data from three studies of Norwegian IDS with 8- to 18-month-old infants (N = 132 infants, N = 182 parents). Parents' speech was recorded during a shared-reading interaction with their infant and with an experimenter (as a control register), ensuring that speech content was matched across participants and registers. Contrary to our hypotheses, we found no evidence that parents emphasized IDS for words attributed as unknown. Furthermore, Bayesian analyses suggested that Norwegian parents do not modulate IDS based on infants' attributed word knowledge. Thus, our findings do not support the notion that parents use IDS as a didactic strategy to teach individual words, underscoring alternative motivations for engaging in such speech. (PsycInfo Database Record (c) 2026 APA, all rights reserved).