After receiving his doctorate, Miller stayed at Harvard as a research fellow, continuing his research on speech and hearing. He was appointed an assistant professor of psychology in 1948. The course he developed on language and communication eventually led to his first major book, "Language and communication" (1951). He took a sabbatical in 1950, and spent a year as a visiting fellow at the Institute for Advanced Study, Princeton, to pursue his interest in mathematics. Miller befriended J. Robert Oppenheimer, with whom he played squash. In 1951, Miller joined MIT as an associate professor of psychology. He led the psychology group at the MIT Lincoln Lab and worked on voice communication and human engineering. A notable outcome of this research was his identification of the minimal voice features of speech required for it to be intelligible. Based on this work, in 1955, he was invited to talk at the Eastern Psychological Association. That presentation, "The magical number seven, plus or minus two", was later published as a paper which went on to be a legendary one in cognitive psychology.