The world population, which surpassed 6 billion last year, is expected to hit 8.9 billion by 2050, according to median estimates from the U.N. Population Fund. Average fertility worldwide has fallen dramatically in recent decades, from five children per woman in the 1960s to 2.7 today, and the fertility in 61 countries, representing 44 percent of the world’s inhabitants, is below the replacement rate of 2.1 live births per woman. But the overall world population continues to rise steadily, in part because global life expectancy has climbed over the past five decades from 46 years to 66 years. Most of the coming population growth is expected to take place in the poor, developing countries that are least able to support it.