Public attitudes about immigration in the U.S. were heavily influenced in the aftermath of the 9/11 attacks. After the attacks, 52% of Americans believed that immigration was a good thing overall for the U.S., down from 62% the year before, according to a 2009 Gallup poll. A 2008 Public Agenda survey found that half of Americans said tighter controls on immigration would do "a great deal" to enhance U.S. national security. Harvard political scientist and historian Samuel P. Huntington argued in his 2004 book "Who Are We? The Challenges to America's National Identity" that a potential future consequence of continuing massive immigration from Latin America, especially Mexico, could lead to the bifurcation of the United States.