While most of the early 19th-century emigrants from Haiti were from the upper classes, persistent conditions of poverty, as well as political unrest, eventually prodded many lower-class Haitians to emigrate as well. Altogether, there have been four periods of major migration to the United States in the history of Haiti: the initial wave at the turn of the 19th century, following the U.S. occupation from the 1915 to 1934, during the 1960s and 1970s to escape the Duvalier regime, and following the 2004 overthrow of Jean-Bertrand Aristide.