Doubling-time

Doubling-Time Definition

Doubling-time is the time it takes for a population of a given size to double. A standard measure in demography, it is nowadays reckoned in decades rather than centuries — a fact that itself testifies to the acceleration of population growth in the modern era.

How It Works

Doubling-time is a function of the rate of growth, and because population increase is exponential rather than linear, even apparently modest growth rates produce strikingly short doubling-times. Demographers commonly approximate the relationship with a simple rule: dividing seventy by the annual percentage growth rate gives the approximate doubling-time in years. A population growing at 1 percent a year thus doubles in roughly seventy years, while one growing at 2 percent doubles in about thirty-five, and at 3 percent in scarcely more than twenty-three. Small differences in growth rates therefore translate into enormous differences in how quickly a population expands.

Significance

The measure is valuable because it renders abstract growth rates vivid and comparable across societies and historical periods. For most of human history, population doubling took many centuries; the demographic transformations that accompanied industrialization, improved food supply, and falling death rates compressed the process dramatically, and in the twentieth century world population doubled within a matter of decades. Rapid doubling-times, characteristic of many developing societies during the peak of the postwar population explosion, raise far-reaching sociological questions about pressure on resources, housing, education, and employment, and about the capacity of institutions to absorb so many new members so quickly. Conversely, where fertility falls to low levels, doubling-times lengthen toward infinity, and attention shifts to the opposite concerns of population aging and decline. The concept is thus a compact indicator of a society’s demographic momentum.