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


the more accurate a projection claims to be, the less trust it deserves - demographer, Joel Cohen


Why 6 billion?

Cloudy crystal ball
Although the general formula for population change is simple, it conceals more untidy details than the room of a 7-year-old boy. crystal ball Since every number in the equation involves a forecast, the outcome is an unsteady pile of projection stacked on assumption.

And we haven't even mentioned the other factors, including such things as literacy, infant mortality and wealth, that influence the desire and ability to have children. How do demographers project population?

The simple-minded approach is to extend the recent population graph a few years into the future. If the trend is already moving upward, you might tweak the curve even more steeply, and vice versa if the trend is dropping. Problem: things change. What went up comes back down -- presumably that's true of population growth as well as the stock market -- reducing the accuracy of this so-called "graph-fitting" technique.

At the other end of the sophistication spectrum are "systems models" which try to include everything that could affect population, such as politics, culture, economy, agriculture and disease. No population can survive, obviously, if it exceeds its food supply or runs up against environmental constraints, so this broader focus is popular among environmentalists.

chart of factors affecting 
population But figuring out the food supply next decade -- let alone how politics will change over 30 years -- is tougher than swabbing the decks of that kid's room. As economists point out, when one product gets rare (and expensive), we substitute more abundant (cheaper) ones.

Agricultural production has been expanding for centuries, and even though the rate has slowed recently, most experts predict continued growth, even as the amount of land per person declines. (Local famine and malnutrition among the poor is another matter. About 840 million people -- one in seven -- are malnourished today.)

Despite their theoretical elegance, system models are too preliminary to do much good, says Joel Cohen, a demographer with the Laboratory for Populations at Rockefeller University (see "How Many People... " in the bibliography).

In cahoots with the cohort
Which brings us to the "cohort-component" method that, Cohen says, is used by every major demographic institution. This method builds on a blindingly obvious fact: only women of child-bearing age can have children.

We read your mind. "No duh!" But don't give up quite yet.

Census data tells how many women will be in childbearing age groups for at least 15 or 20 years down the line. Obviously, girls aged 10 to 14 will be 15 to 19 (in the child-bearing range) after five years. The census data also says how many women of this age are now having children. If we assume this fertility rate will hold for the next cohort of women aged 15 to 19, simple multiplication tells us how many babies they will have.

Although the cohort method also has problems when conditions change, it's the state-of-the-population-forecast art.

Did you want perfection? As Cohen writes, any population prediction is subject to error. "Events in the sphere of human affairs necessarily depend on the conjunction of a very large number of general preconditions and specific factors, any one of which is subject to change. Such events often involve the behavior of a large number of humans, whose individual and collective behavior is notoriously difficult to predict."

One thing doesn't need prediction: some people want to control their fertility -- today.

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