Aunt Alice changes her speaking tone and uses baby-talk phrases when she sees her baby niece. What is this behavior known as?

The behavior exhibited by Aunt Alice is known as child-directed speech. This term describes the unique way adults often modify their speech when interacting with infants and young children. It involves using a higher pitch, exaggerated intonation, simplified vocabulary, and repetitive phrases. These adaptations make the speech more engaging and easier for infants to process.

Child-directed speech is thought to help facilitate language learning by capturing the child's attention and making it more likely that they will respond and engage. Adults instinctively adjust their communication style in this way to create a nurturing and stimulating environment for language development in young children.

The other options do not specifically encapsulate this phenomenon. Adult language modification could refer to any changes adults make in their speech depending on context, while expressive language pertains to a broader range of communication abilities that involve the production of language rather than the specific adjustment of speech towards children. Parental baby-talk, while similar in concept, does not encompass the broader category of child-directed speech used by various caregivers, not exclusively parents.