Throughout most of human history, the world's population has grown gradually. It took thousands of years for the global population to reach one billion people (around 1800). Then, in a little more than a century, the population jumped to two billion (by 1960), and to three billion by 1980. In just twenty years—between 1980 and 2000—the world's human population doubled from three billion to six billion people.

Billboard promoting birth control, China, 1984. (UPI/Corbis-Bettmann. Reproduced by permission.)

The human population explosion during the past century was the result of several factors. Fertility rates remained high, while medical and agricultural advances such as antibiotics, immunizations, clean water, and improved food availability reduced mortality rates—especially among infants and children.

It is difficult to predict how rapidly the human population will continue to increase, due to the many factors that affect population growth. Another important question that scholars ask is "How many people...