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

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Immigrants like these Indians at a Sikh festival in Barcelona are bolstering Europe's stagnant population growth rate. Randy Olson/National Geographic hide caption

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Sharing a hillside with high-rise apartment dwellers, children dance at a shop in one of the squatter communities that ring Caracas, a city of 3 million. Today, one in seven people live in slums. Providing them with better housing and education will be one of the great challenges facing a world of 7 billion people and counting. Jonas Bendiksen/Magnum Photos hide caption

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