The distinction between arithmetic and exponential growth is crucial to an understanding of the nature of growth. Arithmetic growth takes place when a constant amount is being added, as when a child puts a dollar a week in a piggy-bank. Although the total amount increases, the amount being added remains the same. Exponential growth, on the other hand, is characterized by a constant or even accelerating rate of growth.

At a constant rate of increase, measured in percentages, the amounts added grow themselves. Growth is then usually measured in doubling times because these remain constant while the amounts added increase. When the annual rate of increase is 1%, the doubling time will be 70 years. From this fact, a simple formula to calculate doubling times given a rate of increase can be derived: dividing 70 by the percentage rate will yield the number of years it takes to double...