From July 1, 1927, a total limit of 150,000 emigrants was set for admission into the USA. The ratio of the various nationalities within the 150,000 limit would be decided by the existing American population from the respective nationality.
Wives (older than 21 years), parents, and unmarried children (under the age of 21) of U.S. citizens and persons in religious and academic fields could emigrate to the USA regardless of this quota. Immigration from Latin America, as well as the rest of the Western Hemisphere, was also allowed to continue unhindered. Within the quota, immigrants related to U.S. citizens, above the age of 21, or skilled in agriculture received top billing.
This allowed the government to crackdown on the main problem: Italian immigration. Immigrants from northern Europe, particularly Great Britain and Ireland, who had many cultural similarities with Americans, were not significantly restricted by either immigration Act, while Italians and Jews, who were centered in eastern European countries, faced a severe restriction. Italians, in particular, were much less populous in the 1890 census than in the 1910 census, and the false lower numbers allowed the government to set the limit for further immigration much lower than it would otherwise have been.
The effects of this discriminatory policy were immediately clear.
In 1924, more than 59,000 immigrants arrived in the USA from Great Britain, more than 13,000 from eastern Europe, and more than 56,000 from Italy. After the 1924 Immigration Act was passed, the Brits were reduced to just more than 50%―more than 27,000 came to the States in 1925. In contrast, eastern European immigrants dropped by about 88%―about 1,500 came to the U.S. in 1925―and the Italians were reduced by about 89%―only 6,200 came to the States in 1925.
Among non-European emigrants, the 1924 Act also banned immigration from India (including present-day Pakistan and Bangladesh), Japan and China, and the Arab countries. This was thanks to a provision which stipulated that immigrants ineligible to become U.S. citizens could not enter the U.S. as emigrants.
The Immigration Act of 1924 was blatantly inspired by eugenics, and was passed with the intent of "preserv[ing] the ideal of American homogeneity". However, this should be viewed in the light of the period, when eugenics was a common practice for many governments all over the world.
The 1924 Act was superseded by the Immigration and Nationality Act of 1965. This Act was proposed by Representative Emanuel Celler and Senator Philip Hart. Celler had been a vocal dissenter towards the 1924 Act, and had campaigned furiously in the years since it was passed to have it repealed. The 1965 Act changed the entire structure of the National Origins Formula for immigration into the USA.