During the Developmental Speech period from 1959 to 1967, research coalesced around social interaction as a driver of language growth and around the cognitive processing of speech sounds, combining caregiver–child dynamics with early phonological analysis. Studies highlighted caregiver feedback as shaping infant vocalizations, established early diagnostic considerations for speech delays, and mapped typical and disordered speech trajectories. Methodologically, the work relied on naturalistic observation, caregiver–child interaction experiments, and analysis of phonetic structure in the speech of young children to illuminate how speech emerges and changes over time.