been aided by the fact that the United States maintains separate quotas for Mainland China, Taiwan, and Hong Kong. During the late 1960s and early and mid-1970, Chinese immigration into the United States came almost exclusively from Hong Kong and Taiwan creating the Hong Kong American and Taiwanese American subgroups. Immigration from Mainland China was almost non-existent until 1977 when the PRC removed restrictions on emigration leading to the immigration of college students and professionals. These recent groups of Chinese tended to cluster in suburban areas and avoided urban Chinatowns. Third wave (1980s to today) In addition to students and professionals, a third wave of recent immigrants consisted of undocumented aliens, who went to the United States in search of lower-status manual jobs. These aliens tend to concentrate in heavily urban areas, particularly in New York City, and there is often very little contact between these Chinese and those higher-educated Chinese professionals. Quantification of the magnitude of this modality of immigration is imprecise and varies over time, but it appears to continue unabatedly on a significant basis. In the 1980s, there was widespread concern by the PRC over a brain drain as graduate students were not returning to the PRC. This exodus worsened after the Tiananmen Square protests of 1989. However, since the start of the 21st century, there have been an increasing number of returnees producing a brain gain for the PRC. Starting from the 1990s, the demographics of the Chinese American community have shifted in favor of immigrants with roots in mainland China, rather than from Taiwan or Hong Kong. However, instead of joining existing Chinese American associations, the recent immigrants formed new cultural, professional, and social organizations which advocated better Sino-American relations, as well as Chinese schools which taught simplified Chinese characters and pinyin. The National Day of the People's Republic of China is now celebrated in some Chinatowns, and flag raising ceremonies feature the Flag of the People's Republic of China as well as the older ROC flag. The effects of Taiwanization, growing prosperity in the PRC, and successive pro-Taiwan independence governments on Taiwan have served to split the older Chinese American community, as some pro-reunification Chinese Americans with ROC origins began to identify more with the PRC. As pursuant to the Department of Homeland security 2016 immigration report the major class of admission for those Chinese immigrants entering into the US is through Immediate Relatives