Although interpersonal theory is most commonly identified with the work of Harry Stack Sullivan, IPT's pluralistic approach traces its lineage more directly to the work of Adolf Meyer. One of Sullivan's strongest influences, Meyer was a leading figure in American psychiatry in the first half of the twentieth century. He was steeped in the Kraepelinian medical-nosological approach, which was the predominant orientation of American psychiatry in the 1920s. However, he was also influenced by the American philosophical school of pragmatism, which emphasized observable behavior, and by the emerging field of social science.