Robert's development as a linguist proceeded from an early phase of intralingual description and analysis outward toward a comprehensive interlingual perspective focusing on comparison and theory. A large proportion of his work had to do with the Afroasiatic languages, where he made contributions in comparative and historical studies that fundamentally defined that field. He wrote also on the Semitic languages ancillary to his Afroasiatic interests, and he made a special study with considerable publication of his native language, Hungarian.