“It's not just what we say or do, but how we say and do it”

Abstract

Infant-directed speech (IDS) and infant-directed actions (IDA) represent specific behavioural modifications of adults when they communicate with infants and young children. Infant-directed modifications (IDMs) have specific behavioural representations marked by high positive affection, greater expressiveness, simplification, and increased repetition. Both IDS and IDA appear as part of the same larger behavioural system of IDMs. However, so far, studies have analysed the features and functions of these behaviours separately. Compared to adult-directed speech, IDS is characterised by longer pauses, a slower tempo, more prosodic repetition, higher fundamental frequency, longer vowels, repetitive intonation structures, and greater melodiousness. In IDA, compared to adult-directed actions, the amplitude of movements, simplification, and number of repetitions tend to increase as the distance between communication partners decrease. In this review, we draw a parallel between IDS and IDA to show that adults change both their speech and actions in similar ways and that both actions and speech change depending on the infant’s age and/or developmental stage. We discuss possible (biological) mechanisms that elicit the use of IDS and IDA and argue that this specific type of adult behaviour has a unique impact on how infants perceive and process information. Insights on biological, behavioural, and functional aspects of IDMs could provide a new perspective on the importance of early interactions and knowledge acquisition in both typically developing children and those with developmental disorders.