“Even if family planning stopped, habits die hard,” he said. “Overall, our structure is where Japan was in 1992, and our economic waning will be a long-term trend.”

Other demographers strongly disagreed with Mr. Yi.

His findings have not been published in a peer-reviewed journal. Some Chinese experts said that Mr. Yi overstated the problems in the official data, perhaps to fit his longstanding criticism of China’s family planning controls.

“To report that India surpassed China as the largest population on earth, without the announcement of some agency such as the United Nations’ Population Division, or a serious and peer-reviewed study, is not quite professional,” said Wang Feng, a sociologist at the University of California, Irvine, who studies China’s population trends. “The problem with Mr. Yi’s number is that it is based on a hasty and careless calculation.”

He and other experts scoffed at the idea that China had essentially cooked data to create a phantom population almost as big as Henan Province, which has 94 million people.

“I think that the numbers from the National Bureau of Statistics are quite reliable,” said Lu Jiehua, a population researcher at Peking University, referring to the birthrate estimates.

“As well, what would their motive be for faking on a major scale?” Mr. Lu said. “Being totally dismissive is not objective or scientific.”

This debate has broad implications, but it will also be fought over seemingly slight differences in assumptions and data that population scientists use to study trends.