The world’s 6 billionth inhabitant, a boy in Sarajevo, was born today. U.N. demographers chose today as a symbolic marker of the population milestone. Some 78 million new people are born each year, and the equivalent of San Francisco’s population is born every three days. Although the population growth rate is slowing, human numbers have doubled since 1960, and the world population is expected to hit 8.9 billion before beginning to level off in 2050. For their part, Iranian officials are actively pushing free birth control in an effort that has become a model for other nations in the Middle East; the country’s population growth rate has slowed from more than 7 percent a year in the early 1980s to about 1.4 percent per year.