Illegal immigration to the United States is the unlawful entry of foreign nationals into the United States, and the remaining in the country of admitted foreign nationals after the expiration of their U.S. visas or parole documents. Earlier naturalization laws were followed by the 1875 and 1882 Acts prohibiting Chinese immigrants. A 1906 act required immigrants to learn English in order to become citizens, and a 1917 act defined aliens with a long list of undesirables, including most Asians. The U.S. had otherwise nearly open borders until the early 20th century, with only 1% rejected from 1890-1924. A 1924 act established visa requirements and enacted quotas for immigrants from specific countries, especially targeting Italians and Eastern Europeans.
In response to the September 11 attacks, the U.S. Department of Homeland Security was formed under the Attorney General, and it absorbed the Immigration and Naturalization Service in 2003. Estimates in 2015 put the number of unauthorized immigrants at 11 million, representing 3.4% of the total U.S. population. The population of unauthorized immigrants peaked in 2007, when it was at 12.2 million and 4% of the total U.S. population. Since the Great Recession, more undocumented immigrants have left the United States than have entered it, and illegal border crossings are at the lowest levels they have been in decades. In 2014, unauthorized immigrant adults have lived in the U.S. for a median of 13.6 years, with approximately two-thirds having lived in the U.S. for at least a decade. In 2012, 52% were from Mexico, 15% from Central America, 12% from Asia, 6% from South America, 5% from the Caribbean, and another 5% from Europe and Canada.
Research shows that illegal immigrants increase the size of the U.S. economy, contribute to economic growth, enhance the welfare of natives, contribute more in tax revenue than they collect, reduce American firms' incentives to offshore jobs and import foreign-produced goods, and benefit consumers by reducing the prices of goods and services. Economists estimate that legalization of the illegal immigrant population would increase the immigrants' earnings and consumption considerably, and increase U.S. gross domestic product.
There is no evidence that illegal immigration increases the rate of crime in the United States. There is scholarly consensus that illegal immigrants commit less crime than natives. Sanctuary cities – which adopt policies designed to not prosecute people solely for being in the country illegally – have no statistically meaningful impact on crime or reduce the crime rate. Research suggests that immigration enforcement has no impact on crime rates.