Americo-Liberians were credited for Liberia's largest and longest economic expansion in the early to late twentieth century, especially William V. S. Tubman, who did much to promote foreign investment and to bridge the economic, social, and political gaps between the descendants of the original settlers and the inhabitants of the interior. Most of the powerful old Americo-Liberian families fled to the United States in the 1980s after President William Tolbert was assassinated in a military coup.