The average time it takes for a particular population to double in size is known as its "doubling time." This statistic is a rough indicator of the fertility of a population. Factors responsible for variation in reproductive rates are complex and imperfectly understood, but the world's doubling times have become progressively briefer throughout recorded history. It took several hundred years for the world's population to double from 1 million to 2 billion, less than one hundred years for doubling from 2 billion to 4 billion, and less than fifty years for doubling from 3 billion to 6 billion. It is believed that improvements in public health are the main cause of these shorter doubling times. Within individual nations, doubling times relate closely to total fertility rate, and they can vary considerably. As of 2000, several European countries had doubling times of hundreds of years, while in Kenya, the doubling time was less than twenty years.
JOHN M. LAST