Social Darwinism did indeed affect American immigration policies in the 19th and 20th centuries. In 1882, the Chinese Exclusion Act restricted Chinese immigration to the United States: for ten years, Chinese laborers were not allowed to enter the United States. After the law expired in 1892, Congress extended the restrictions on Chinese immigration in the form of the Geary Act. Under the Geary Act, immigrants of Chinese descent had to obtain certificates of residence or face deportation.
Meanwhile, Charles Darwin's "survival of the fittest" theory became the basis of a pseudo-scientific movement called eugenics. Eugenicists believed that the human race could be rehabilitated or improved by a process of selective breeding.
Since those of northern European stock were said to be superior in terms of intelligence, emotional stability, and physical endurance, eugenicists believed that America should admit only immigrants of European descent. The term "eugenics" was first coined by Sir Francis Galton, Darwin's half-cousin. Shockingly, two of the main supporters of the eugenics movement were Presidents Theodore Roosevelt and Woodrow Wilson.
In 1924, eugenicists were successful in helping to pass the Immigration Restriction Act of 1924. The law created quotas for immigration to the United States. Immigrants from northwest Europe and Scandinavia were allowed to enter in larger numbers than those from eastern and southern Europe. Accordingly, the 1924 law remained in place until it was repealed by the 1965 Immigration and Nationality Act.