Claims that India may already have overtaken China as the world’s most populous nation have sparked consternation among demographers.

The claims were made on Monday by Yi Fuxian, a University of Wisconsin-Madison academic who has spent years campaigning against Beijing ’s draconian family-planning laws, and picked up by newspapers in both China and India.

Speaking at a conference in Beijing, Yi said he was convinced that over the last 26 years Chinese statisticians had overestimated the country’s population by about 90 million, partly by inflating rates of fertility. As a result, China’s population at the end of last year would have been 1.29 billion.....