The earliest Iranian people in the U.S. were mostly young trainees who worked as medical interns or residents. Some established themselves to continue practice beyond the residency stage. Their motives to extend their stay in the United States were more for professional than economic reasons. Researchers from Johns Hopkins University in 1974 reported, in the "Journal of American Medical Association", that, in 1971, the number of Iranian physicians in the U.S. was 1,625. The authors further studied the causes for immigration by sending questionnaire to all Iranian MDs in the United States. According to the 660 respondents, the main reasons for migration were mandatory two-years' military service, low salaries as compared to the United States, expensive housing, and socio-political reasons.