Flagging prospects and a lumbering U.S. bureaucracy are pushing talented immigrants back home to India and China after 40 years of a major brain drain, the author of a new study says.

"Over the past four decades, India and China suffered a major ‘brain drain’ as tens of thousands of talented people made their way here, dreaming the American dream," says Vivek Wadhwa, author of the study.

"But burgeoning new economies abroad and flagging prospects in the United States have changed everything," notes the Indian-American technology entrepreneur turned academic in an article in the Washington Post.

"And as opportunities pull immigrants home, the lumbering U.S. immigration bureaucracy helps push them away," said Delhi-born Wadhwa, a senior research associate at Harvard Law School.

Wadhwa recently surveyed 1,200 Indians and Chinese who worked or studied in the U.S. and then returned home.

"When smart young foreigners leave these shores, they take with them the seeds of tomorrow’s innovation," he said. "Almost 25 per cent of all international patent applications filed from the U.S. in 2006 named foreign nationals as inventors."

Immigrants founded a quarter of all U.S. engineering and technology companies started between 1995 and 2005, including half of those in Silicon Valley. In 2005 alone, immigrants’ businesses generated $52 billion in sales and employed 450,000 workers.

"Yet rather than welcome these entrepreneurs, the U.S. government is confining many of them to a painful purgatory," Wadhwa said.

As of Sept. 30, 2006, more than a million people were waiting for the 120,000 permanent-resident visas granted each year to skilled workers and their family members.

An analysis of the 2000 Census showed that although immigrants accounted for only 12 per cent of the U.S. workforce, they made up 47 per cent of all scientists and engineers with doctorates.

What’s more, 67 per cent of all those who entered the fields of science and engineering between 1995 and 2006 were immigrants.

"Their talents will benefit nations such as India, China and Canada, not the US. America’s loss will be the world’s gain," said Wadhwa.