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.