Medical journal The Lancet recently published research by the University of Washington suggesting that population decline could be more rapid than previously thought, especially in the world’s most populous nations China and India.

The researchers assume that world population will peak already just after the middle of the century, earlier than projected by the U.N. Population Division. They pointed out that models of populations growth have proven to be very stable while those dealing with population decline were much less reliable.

In their base scenario, researchers assumed growing access to education and contraception for women would catapult Indian and Chinese fertility below replacement levels quickly, leading to population levels of just 1.1 billion and 731 million people in India and China in 2100, respectively.

The researchers did not see the same factors at play in most African nations, where population growth would continue to 2100 and beyond, according to the model. This would make Nigeria the second-largest nation on Earth ahead of China by 2094.

The U.S. would remain the fourth-largest nation in 2100, despite below-replacement levels of fertility. Here – similar to the situation in Canada and Australia – positive net migration would keep population levels semi-stable.

Also by 2100, current growth nations Indonesia and Pakistan would have entered a slight population decline per the model, with the Democratic Republic of the Congo and Ethiopia overtaking them in size around the turn of the new century.

The U.S. will remain the fourth-largest nation in 2100, despite below-replacement levels of fertility.