The crowded scene on "Black Friday" at Macy's in Manhattan. The U.S. population is projected to hit 400 million in 2051, Census says, up from 321 million in 2015. Stan Honda /AFP/Getty Images hide caption

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Indian schoolchildren write English alphabets on slates at a primary school outside Hyderabad in June. India is on track to overtake China as the most populous nation in just 16 years. Noah Seelam/AFP/Getty Images hide caption

itoggle caption Noah Seelam/AFP/Getty Images