Emigration is a major part of contemporary Puerto Rican history. Starting soon after World War II, poverty, cheap airfares, and promotion by the island government caused waves of Puerto Ricans to move to the United States mainland, particularly to the northeastern states and nearby Florida. This trend continued even as Puerto Rico's economy improved and its birth rate declined. Puerto Ricans continue to follow a pattern of "circular migration", with some migrants returning to the island. In recent years, the population has declined markedly, falling nearly 1% in 2012 and an additional 1% (36,000 people) in 2013 due to a falling birthrate and emigration. The impact of hurricanes Maria and Irma in 2017, combined with the unincorporated territory's worsening economy, led to its greatest population decline since the U.S. acquired the archipelago.