Immigration to the United States is a complex demographic phenomenon that has been a major source of population growth and cultural change throughout much of the history of the United States. The economic, social, and political aspects of immigration have caused controversy regarding ethnicity, economic benefits, jobs for non-immigrants, settlement patterns, impact on upward social mobility, crime, and voting behavior. On a per capita basis, the United States lets in fewer immigrants than half the countries in the OECD. Prior to 1965, policies such as the national origins formula limited immigration and naturalization opportunities for people from areas outside Western Europe. The civil rights movement of the 1960s led to the replacement of these ethnic quotas with per-country limits. Since then, the number of first-generation immigrants living in the United States has quadrupled, from 9.6 million in 1970 to about 38 million in 2007. Nearly 14 million immigrants entered the United States from 2000 to 2010., and over one million persons were naturalized as U.S. citizens in 2008. Since the per-country limit applies the same maximum on the number of visas to all countries regardless of their population, it has had the effect of severely restricting the legal immigration of persons born in Mexico, China, India, and the Philippines – currently the leading countries of origin of immigrants to the United States. Family reunification accounts for approximately two-thirds of legal immigration to the US every year. , 66% of legal immigrants were admitted on this basis, along with 13% admitted for their employment skills and 17% for humanitarian reasons. Migration is difficult, expensive, and dangerous for those who enter the US illegally across the Mexico–United States border. Virtually all undocumented immigrants have no avenues for legal entry to the United States due the restrictive legal limits on green cards, and lack of immigrant visas for low skilled workers. Participants in debates on immigration in the early twenty-first century called for increasing enforcement of existing laws governing illegal immigration to the United States, building a barrier along some or all of the U.S.-Mexico border, or creating a new guest worker program. Through much of 2006 the country and Congress was immersed in a debate about these proposals. few of these proposals had become law, though a partial border fence had been approved and subsequently canceled.