The period presents a dual focus on investigating gestural communication as a potential linguistic substrate and on treating American Sign Language as a fully fledged linguistic system within Deaf communities, alongside cross-species comparisons with primate gesture. Research relies on controlled training, observational analysis, and sociolinguistic examination of sign usage, reflecting an integrative approach that binds human language inquiry with primatology. Historical Significance: The era clarifies the boundary between symbolic language and spoken language, and introduces rigorous methods for assessing genuine sign use in nonhuman primates, while also introducing the concept of sign language diglossia to address variation between formal and informal signing.