The year of six billion
Math of population
Problematic projections
Unsatisfied demand
Was Malthus right?


a woman's age at childbearing greatly affects the rate of population growth


Why 6 billion?

Produce. Reproduce
We'll spare you the gruesome details, but here's what we've divined about the math of population growth.

population change=(births + immigration) - (deaths + emigration) First off, the population of an area depends on births, deaths and the number of people moving in or out.

If you were as simple-minded as we Why Filers, you'd figure that was it. But statisticians don't like to add --- they greatly prefer sophisticated math, and rely on percentages rather than absolute numbers to express population growth. Conclusion? The math gets kinda complicated kinda fast, professor. We'll spare you the gruesome details, but just remember that the rate of increase depends on the number of children per woman, and on the length of generations -- her age at the first baby.

Thus, says Donald Waller, a University of Wisconsin-Madison professor of biology and ecology, if all women had three kids with a 15-year average generation time, the rate of population growth would be 2.7 percent. If the average spacing were 30 years, the growth would drop in half -- to 1.35 percent.

Waller notes that exponential growth (compound interest to economists) is characteristic of living organisms. One bacterium divides into two, which make four, then 8, 16, and pretty soon you have a whole lakeful of bacteria.

At today's average rate of global population increase, a couple who had children 12,000 yrs. ago would now have 5.3 x 10 83 descendants! It's the same with people, except, as Waller notes, we're a species that has the capacity to see into the future, and to plan for our children's welfare.

Doubling time
Here's a simple rule for population growth: To find out how long it takes for a population to double at a certain rate, divide 70 by the percentage increase per year. Be sure to use a whole number for the percentage. If a population is growing three percent per year, divide 70 by three, not 0.03.

animated growth map


How do these factors translate into population predictions?

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