He and others like him left the United States with fewer regrets than some Americans might assume. While he was courted by a clutch of top American universities and rose swiftly through Princeton’s academic ranks, Dr. Shi said he believed many Asians confronted a glass ceiling in the United States.

Rao Yi, a 47-year-old biologist who left Northwestern University in 2007 to become dean of the School of Life Sciences at Peking University in Beijing, contrasts China’s “soul-searching” with America’s self-satisfaction. When the United States Embassy in Beijing asked him to explain why he wanted to renounce his American citizenship, he wrote that the United States had lost its moral leadership after the 9/11 attacks. But “the American people are still reveling in the greatness of the country and themselves,” he said in a draft letter.

These scientists were not uniformly won over by the virtues of democracy, either. While Dr. Rao said he hoped and believed that China would become a multiparty democracy in his lifetime, Dr. Shi said he doubted that that political system “will ever be appropriate for China.”

As a Tsinghua student, Dr. Shi joined the 1989 pro-democracy protests in Tiananmen Square. As a registered Democrat in the United States, he participated eagerly in elections. “Multiparty democracy is perfect for the United States,” he said. “But believing that multiparty democracy is right for the United States does not mean it is right for China.”

Yet the re-entry to the politicized world of science in China can be challenging. Some scientists with weaker résumés have shunned returnees. In its biennial election of academicians last month, the Chinese Academy of Sciences, China’s highest advisory body on science and technology, passed over Dr. Shi and Dr. Rao. It also did not recognize Wang Xiaodong, a well-known Howard Hughes Medical Institute investigator who recently became a director of Beijing’s National Institute of Biological Sciences while remaining a full-time professor at the University of Texas Southwestern Medical Center in Dallas.

The tension has spilled over into the Chinese blogosphere, where Dr. Shi has been attacked as insincere and untrustworthy. In a posting in 2008, Liu Zhongwu, a professor of science and engineering at South China University of Technology, said that Dr. Shi should be excluded from any projects that touch on China’s national interests. “Bear in mind, he is a foreigner,” he wrote.