It took all of human history for the world’s population to reach its first billion in 1800. Now we add a new billion nearly every dozen years. Over the next half century, 98 percent of that growth will take place in our planet's poorest regions, and as the global total swells to nearly 9 billion by 2050, the social and environmental strains will be enormous. In Japan, Europe and Russia, birth rates are shrinking and the population is ageing, but in parts of India and Africa, more than half of the still growing population is under 25. The surprising conclusion is that the world population is now careening in two dramatically different directions.