Don’t bet on the stratospheric rise of the economies of China and India continuing long term, argues Larry Summers, an economist who serves as President Emeritus of Harvard University.

History shows countries with “abnormally rapid growth” simple don’t have staying power — and when those countries’ economies do slow, the change will have major repercussions on the rest of the world, he co-wrote in a paper with Harvard professor Lant Pritchett and published by the National Bureau of Economic Research this week, according to CNN Money.

Summers and Pritchett argued that massive growth as shown by China and India are typically succeeded by a period of mediocre growth, and that there will be a period of regressing to the mean. Economic forecasters often ignore this possibility, basing future projections on the most recent growth results.

Once that growth does stop, the declines will be sudden and massive, Summers warned.

He noted the 2008 economic downturn in the United States was not predicted by economists, and the risk of downside scenarios aren’t properly taken into account. He also pointed to a 1961 textbook that predicted the USSR would pass by the United States by the 1980s, but instead the Soviet Union collapsed.

China, on the other hand, has been growing heavily since 1977, a very long period of rapid growth, making it ripe for a downturn.

There has also been a push within China to move toward democracy, and that alone could hammer the nation’s economy, as nearly every country that has done so after strong growth typically sees its economy head south within the next decade, Summers wrote.

If China could somehow maintain its growth over the next 20 years, its economy would be worth $60 trillion — triple the size of the United States. If Asia slows down considerably as the paper posits, it would stand at around $20 trillion, a considerable difference.