During the 1970s, when public debate over uncontrolled world population growth was especially agitated, some unknown writer claimed that "three-quarters of all the people who have ever been born are alive today." That arresting but erroneous statistic was repeated again and again until it became accepted as fact, adding to the catalog of urban legends. For more than two decades, demographer Carl Haub of the Population Reference Bureau, a nonprofit population research group in Washington, has fielded calls from people trying to confirm the claim. Even today, sporadic requests continue.
At first, it might seem that the question of how many people have ever lived on Earth is impossible to answer. However, there is enough information available for demographers using reasonable assumptions to make a pretty good "guesstimate," Haub said.
The answer, according to Haub's calculations, is that a ...