Barth had originally planned to submit the manuscript of his "Principles of Social Organization" as his Ph.D. dissertation, but was unsuccessful in doing so. He continued graduate study, moving to Cambridge, England to study with Edmund Leach, whom he had previously worked with at the LSE. For his PhD, Barth conducted fieldwork in Swat, Pakistan; his completed dissertation was published in 1959 as "Political Leadership among Swat Pathan". Shortly afterwards he was part of a UNESCO study of pastoral nomadism, which focused on the Basseri in what is now Iran. From this work, he published the 1961 monograph "Nomads of South Persia".